I never had any problem getting 2-430s to play nice in a one box, but haven't tried it since the 285 series nvidia drivers. I never tried 3. The psu should be able to power 3, they're about 50w per card. Dell mbs can be problematic sometimes, so I see that as a more limiting factor than the power supply.
Edit: It would also be advisable to be able to check the amp draw at the wall before adding the 2nd and/or 3rd card. Then again, you might want to believe nv, in which case the 22a rail is only enough for 1 430.