That's not enough horses to carry guitar and bass; use small stage amps for those instruments to balance with the drummer. Then use the Samson for vocals only.
Also, the Samson's spec sheet shows 4-ohm min operation. If you're using typical 8-ohm PA cabinets, you can only run two of them using the S63's built-in amp. (You could use the monitor out to feed either powered monitors or an external power amp that would drive your monitors.)
The keys to having a decent night:
- Keep stage volume low. (If your drummer and guitarist can't play quietly, you're in for a long night.)
- EQ and place monitor for best intelligibility before feedback. (Look up the spec sheet of your vocal mics to see whether the best monitor placement is directly in front of the mic or at an angle.)
- Use vocal effects sparingly,especially if you end up running the same output to one main and one monitor. (The more FX on your vocals, the less intelligible they'll be--meaning the more gain you'll need to hear them.)
In short, you'll need to be realistic about—and adjust to—the modest volume you can wring out of your PA.