When you're looking for a 3v3 Strategy in Starcraft 2, it's important to realize how difficult it really is to wipe out three separate opponents when they have the defender's advantage. Thus, you really need an overwhelming force that's strong against some of the toughest units in the game, which the opponent could field en masse.
If you've read my other 3v3 strategy, you'll note that specializing and making the big damage-dealing units for your team is a great way to use Protoss. If you can get a teammate to feed you gas, you can get Colossus or Immortals out before the opponents will have enough of the counter units. Chrono-boost allows you to get these units out even more quickly as well.
This unit hard-counters most big units, and many tough, mass-produceable units like Roaches and Stalkers. Let your allies go with Colossi or Siege Tanks or Banelings/Infestors to counter mass-zealot or marine, while you annihilate pretty much everything else. Mix in a few anti-air units, and you have quite the killer army.
Shutting Down Mass Muta
The other function of the Protoss player is to shut down Mass Muta strategies by threatening mass Phoenix. Mass Phoenix can actually be better than you might think, even in straight-up army battles, because they can lift and kill high-dps units, such as Hydras, Templar, Ghosts and Infestors. While this requires a lot of micro, it CAN turn the tide. However, you should avoid such scenarios whenever possible, and instead use Phoenixes to snipe Overlords or other scouts, to discourage further Muta production, to maintain air dominance, and to protect against drops.
The Protoss player also has another item in his arsenal that can be overlooked: Photon Cannons. This single building can be placed virtually anywhere (except on creep), and can also defend against nearly anything (or at least slow it down). Use Photon Cannons liberally at expansions (yours and teammates'), and you'll find that enemy harassment dries up quickly. What you must be careful of is not to overbuild cannons before you have the economy to support it, otherwise you won't have much of an army, and you or your allies can be hit where you don't have cannons. Cannons can't counterattack, either. However, because team games tend to be longer than 1v1's, your side will eventually have the resources to cannon up key expansions and choke points.
I also think Chargelots are among the best meat-shield that you can get in almost any race. They are fairly tough, small enough to be fielded in large masses, aren't worth using Neural Parasite on (unlike Archons) and only cost minerals. Better still, they move in quickly and chew up other players' meat shields quite well (even Roaches, which are usually thought to be their counter unit). Because of the mass of units in a big 3v3 battle, the Zealots will not be able to be kited, and will do great damage for their cost, especially with attack upgrades. However, Zealots can also suck when faced by enemy Colossi and Infestors, so their use can be limited by the opponents' unit composition. Yet, in 3v3, it's not as likely that the enemy army will be able to micro well, if only because of the sheer amount of stuff on the screen, and then Zealots can perform uite well (it doesn't matter if they are Fungal'ed if they are already doing damage!).
Protoss also have the most annoying unit in team games: Dark Templar (DTs). Almost every player has lost a game to the sudden appearance of 5+ DTs in their main base, and a team game gives a Protoss player the ability to mass Dark Templar without the risk that their own base will be flattened by a counterattack if the DTs are spotted early. If you aren't facing any Protoss opponents (i.e no enemy Observers), I recommend throwing in even 4-5 DTs at the front of your army. They will prevent enemy melee units from reaching units directly behind them, and many opponents won't realize they even exist - stuff is dying so fast that they won't notice the extra damage from the extremely-powerful DTs.
Mass Blink Stalkers
Last, you shouldn't ignore the possibility of going mass blink-Stalkers. Stalkers are a fast unit with decent damage, and should be used in packs to continually harass enemy locations. Because they can stand up to or run away from many enemy units, and have the speed to run away from many more, Stalkers are great for harassment. I recommend accompanying them with an observer, so they can blink to high ground when pursued by their one counter: mass upgraded Speedlings.