The Answer by Resorath was almost right, but what you want to do in bronze league is always focus on macro above micro.
To this end the protoss death ball should win you most bronze level games against MMM. The difference is that you want to focus on units that don't require as much work on your part. Sentries and high templar require you to use their spells quickly and correctly to get the most out of them. Even immortals require you to target fire armored units. Instead go with units that work well on the attack move command and soak up a bunch of damage.
Colossi - you'll want 2-3 at least and the range upgrade
Stalker - don't worry about the blink upgrade
Zealot - do get the charge upgrade
If you can get the +1 armor upgrade do so, it is far better than having +1 attack versus terran.
This build does require some time in order to build up your colossi and army count. You will want to delay your enemy's attack as long as possible. A mothership core (if you have heart-of-the-swarm) with photon-overcharge on a nexus can halt very early rushes. Some scouting or grabbing of a watch tower can let you know when your enemy is going to move out. I would also recommend trying to counter attack your enemy if he tries to move out before you are ready, but this is more difficult to do at bronze league. If you are finding that you are dying before you can pull this build off try cutting the zealot charge upgrade and armor upgrade. If that is still not giving you enough time try building only zealots to start, using all your gas for colossi and the range upgrade.