Two pieces of advice here.
1. A group of 5 or 6 can handle level+3 encounters usually without casualties. But for a group of 3 there is very little redundancy, and hard encounters become extremely swingy. So use Level+2 as the top difficulty, unless you are using waves (see #2).
2. Use waves, and use minions. Delayed entry for a creature or two allows characters to gauge the difficulty of an encounter without that initial onslaught, or the chance of a bad initiative quickly turning an encounter in the monsters' favor. Minions also help moderate the output of damage, and make encounters feel more full, while still dieing in droves.
Encounters have to be tailored to a small group for optimal "fun". A group of 3 stealthy strikers might be a lot of fun to play. Massive battles with numerous opponents is not going to be their thing, but they sure will be able to take down the occasional elite. A group of 2 defenders and a melee leader will have fun wading through lower level skirmishers, brutes, and minions and will be able to withstand a good bit of punishment. But a regular encounter design with soldiers in front and hard to get to artillery/controllers in the back may spell death for them, as they won't be able to get through the front line quickly enough and may be torn apart by the ranged attacks.