The question is: For general raiding, will i always need a Tanker +
Healer + Dps's in the party ?
To answer this question the answer is No, you do not NEED the holy trinity for dungeons/skirmishes.
There's enough dodging mechanics that you can take almost no damage for entire fights, so you can do 5 DPS a dungeon if you like, but to be fair one of your DPS is going to be running around the whole time with agro constantly trying not to get hit, or drinking alot of potions so a tank is helpful here.
A full group of healers (or tanks) are going to take forever to do a dungeon just because there damage is lower.
When you use the queue system, most of the time it trys to give you tank/heals/3 dps. For this it takes about 45mins to an hour to get into one as a dps. As a tank it's fast and healer it can take some time but not as long as dps.
All dungeons can be entered solo or with your own personal party without The queue system so you can do it with whatever type of group you want.
Most classes use potions for healing, Clerics have personal and group/target heals, tanks get 1 or 2 skills that give back health. Other dps classes I don't think get any (I could be wrong), but you can get items with "Life Steal" on them which will give you health back with each hit, also you can get items with "Regeneration" that will give you health every 2 seconds or something like that.
Normally takes about 45mins - 1 hour.
lots of mobs laying around
resources and other hidden areas can be found to get treasure
1 main boss
Take about 15-20 mins per, (normally) you are taking into an instance with 1 area, 1 main boss 1 sub boss and some mobs.