Question basically states it. I have a lot of problems beating Terran when they either do MMM(Marine Maurader Medivac) with tanks or even just marines and tanks. Tried banelings and infestors, and quite a lot of other strategies so any help would be awesome
So the first problem you have is that these are the units you've listed:
If I were to present a different list of units you might say Zerg is unbeatable:
So instead of trying to deal with everything a Terran player might do, let's break BioMech down into stages and look at what you can do at each stage.
BioMech usually starts as a 3 Rax opening (though it can starts a 5 Rax Reapers, or Early Hellion Harassment). So your early game should largely be based around dealing with Marine/Marauder forces. There are a number of good strategies to dealing with MM, but the most effective is usually Banelings.
Banelings do a very heavy amount of splash damage and since MM tend to
Alternately, if you want to skip Banelings and go for early Lair, a Hydra/Speedling force can be every effective if you can flank. This usually involved engaging at a choke (where your Hydra have better surface area) and running Speedlings around the back of his troops to prevent escape. Good Terran players have a "feel" for the timing on a Marauder reload and can usually micro Marauders without any damage loss. This is why its so important to get the Speedlings behind the MM Ball.
The natural progression of 3 Rax is into MMM. The reason for this is that Stim is a very dangerous tool and if your opponent forces you to Stim at the wrong time, that's a good chunk of HP missing (especially from the high dps Marines). Medivac's largely deal with this problem and increase the effectiveness of Stim. Since Stim is so powerful you'll almost immediately feel when this is happening.
You deal with an MMM much the way you deal with an MM force, there is little difference here. It does become worth starting to get Roaches as a meat shield, and I cannot overstate how effective Infestors can be at this stage. Infestors not only do heavy damage to bunched units, but will also prevent his Stimmed army from getting away from your slower Roach/Hydra. The other thing to watch for is try to engage on creep. Since he'll be using Stim more liberally, you don't want your units getting kited.
If your opponent sees a lot of Banelings he may instead go early Siege Tanks instead of Medivacs. If you force your opponent to do this, there is a good chance you can pull out a win. Since this is the time he's largely switching from 1 base to 2 base his production won't be in place to support a MMTank build. As a result he's going to cut production on something, usually Marauders. Depending on what this something is, you can exploit this for a much easier win. If he cuts production on Marauders, then you can flood him with Roaches. If he cuts Marines, Speedlings or Doom Drops will spell the end for him (as his army will be tiny).
Early Tanks are usually a knee jerk reaction to damage by Banelings. As a result, about 75% of the time you should be able to roll him as his strategy will be in disarray. The remaining 25%, stay calm, scout him, and look for what you can exploit.
Eventually he'll transition from Early Tank or MMM into BioMech. By now you're getting into late game and will have Tier 3 units at your disposal. If you're playing well you should be sitting on 4 Bases to his 2, and this will be a piece of cake. Since you asked this question I'm guessing you're not playing well at this point, which usually means 3 bases to his 2. If you find yourself in a situation of 2 of your bases to 2/3 of his, you need to seriously reconsider when to expand.
At this point you want to start transitioning into your end game strategy. Usually this means: Ultralisk/Crackling, or Corruptor/Broodlord/Roach, or some combination of end game units. The end game part is key. If you find yourself against a large MMM ball supported by Tanks and you're still running around with Baneling/Roach, you've essentially forgotten to tech while your opponent hasn't. To this end: Siege Tanks die quickly to Ultras, BioMech has a lot of trouble with Broodlords, etc.
Now if he rushes BioMech and cuts production of his army, there is a chance you could see 1-2 Tanks/1-2 Medivac/A handful of Marines. This means he's overspent on Tech and you can exploit this for victory. In a situation like this, I can not over state the effectiveness of Speedling/Baneling. If you get those Banelings in there it'll clean his Marines, and your Speedlings will mop up. Since his teching means he doesn't have a ton of Infantry building structures (and other units take much longer to build), you should be able to get your Speedlings from the battle, into his mineral line VERY quickly. A 1:1:1 opening can be effective against Zerg, but if he's using it to go BioMech he's throwing that effectiveness away (as opposed to going Viking/Hellion harass).
Remember, No man can do all things so don't worry so much about how to counter A,B,C,D and worry more about, how do I use what I have to deal with what he has.
You can watch this replay. Its really very exciting and shows how hard it is to beat this type of terran play.
Zerg was able to win only after creating some Brood Lords
Banelings are great early on against marines.
Vs Marine and Tanks it is very easy. Make a ton of zerglings WITH SPEED (zerglings without speed are worthless). Morph a few into banelings. How many exactly depends on the size of your opponents army. Be sure to have a few zerglings left to surround tanks.
I would first send the lings, closely followed by the banelings so the first soak up the first tank hits.
Another strategy vs marines with very few tanks would be roaches. Roaches totally own marines. If you can manage to have enough to survive a few tank hits then you should be ok.
If your opponent has marauders then it gets more complicated. My main strategy against this is to mass roaches and zerglings enough so that I can expand and keep myself safe. Meanwhile, I transition to ultralisks. Ultralisks can't be slowed by the marauders so they will totaly chew MMM while your roaches/zerglings get to them.
Some players will try to go infestors to deal with the bio ball by using fungal growth (units cannot move + dmg over time). While this usually works versus Marine/Marauder, a large number of medivacs will kinda counter this. It might not be instant win with fungal growth but it is definately a good addition to roaches/zerglings.
Some things you should not make (can win but are more risky):
Mutalisks since marines are cheap and counter them well. Thors are a total show stopper too.
I've had troubles with this too.
But I've had a lot of success by diversifying my unit composition. The Terran is using multiple unit types, and so you must too to defend against this.
They're using 4 unit types here, and you will probably need as many.
So a big bio-ball of ling/baneling/roach/mutalisk will be able to hold up against an MMMT group. Roaches to soak up the damage to allow the lings/banelings get close. Mutas to pick off the tanks and medvacs.
And actually, you can probably defend it will if you pick any combination of 4 Zerg types.
Like say, Hydra/Roach/Baneling/Ling...etc...
You mentioned that you're using infestors... fungal growth + a nice unit comoposition will really turn the tide. Also, if you have more than 1 infestor, you can spam infested terrans during the engagement. Infested terrans are like a +2 unstimmed marine.