I've done [Heroic 4] missions with 4 DPS with NO healing abilities and NO CC (ex: DPS Vanguard, 2xSentinels, Guardian). They have been some of the most fun I've had in this game.
It just takes some creative thinking.
You know that group with 2 strongs and 3 elites? 2 focus 1 strong, 2 focus the other strong and then all run out. Come back. Focus 1 elite and run out. Come back. Focus 1 elite and run out. Come back. Focus last elite. Done.
The Champion? 1 DPS runs in and aggros him and starts to run/LOS (Line of Sight) him. The others start to DPS. When aggro switches to another DPS, that DPS starts to run/LOS the champion and the one that was running/LOSing starts to DPS. Keep switching aggro/DPS roles and BAM! dead champion.
Every class has defensive cooldowns: use them. Every class has a stun: use them (even a 1 second stun is amazing). Don't be afraid to run away from a melee boss if he is focusing you down: while he is chasing you the others will be DPSing him down. If a boss is ranged, LOS him: while he runs to get into a position where he can attack you again the others will be DPSing him down.
If one (or more) of your team mates has a CC (even a short term one) or healing abilities (Operative/Smuggler, Sage/Sorcerer, Commando/Mercenary), even untalented, it becomes even easier.
Just be creative and you'll be amazed at just how much you can achieve with a non-ideal (1 tank, 1 healer, 2 DPS) group. People just need to think "outside the box".