1. Beware of support characters. With three PCs, it is likely that every PC will have to step up into the starring role frequently. If even one out of three PCs does not have a way that he can win the battle, then, when the chips are down and that PC needs to come through, the party will probably lose.
There is another way that purely support characters will harm the party: What do they do when the party does not need support? A cleric with no combat ability can cast bless, but when he doesn't have a bless spell or doesn't want to waste one and no-one needs healing, he doesn't have anything useful to do. A more traditional cleric, on the other hand, can at least make himself useful by beating things with his mace. Now, the long term effect of the pure support character is that, since the party only has 2/3 of the effective offensive actions that another party might have, it will take them longer to defeat even trivial foes. Therefore, the enemies will survive to get more actions than they might otherwise get and will inflict more damage on the party.
Note that bards, clerics, and even marshals are not necessarily pure support characters though they are often created to fill that role. Even a single class bard who focuses his abilities in that direction can be a competent melee combatant in his own right, if a little squishy. The buff and bash cleric is a pretty standard archetype. And Marshal mutliclasses very nicely with any of the fighting classes.
2. Consider Leadership. Three characters and a cohort (or three) are much more effective than three characters by themselves with one more feat each.
3. Re-consider the role of NPCs. I'm not talking about DMPCs here, but the NPCs that the PCs meet in the course of their adventures. Maybe the sister of the dead elf they find swears an oath of vengeance against his killers and accompanies the party until she cuts the still beating heart out of the body of the goblin leader. After that, her oath is fulfilled and she returns to her people. Adventuring isn't her profession, she was in it for blood not money. Or perhaps there is a Ruby Knight in the temple of Wee Jas when the PCs come to inform them of an undead threat under the mines.... and he offers to accompany the PCs to deal with this threat. Of course, all NPCs won't be trustworthy and most of the time, their agendas will intersect with the PCs' agendas rather than run parallel to them. Done properly, this could have the effect of involving the PCs more deeply in a living world rather than having a group of PCs stand separate from the world.
4. For the players, be able to fill multiple roles. A multiclass fighter/wizard (done correctly) does not exactly fill either role (rather, he has a role of his own) but he does bring a lot of capabilities to the party that the straight-up fighter would not. Likewise, a Shadowbane Stalker isn't quite as good at clericing as a single-class cleric, but is good enough to get by and can bring a lot of a rogue's trap and stealth abilities to the table too.
5. For the DM: consider a more generous stat generation method. If your standard is 28 point buy, maybe 32 point buy will give the PCs a little extra edge. If you do 4d6x6, drop the lowest, maybe adding a wildcard roll will give the PCs the extra edge they need.
