three blade props ? bad idea on a bipe with clearence issues?

Not if you have an engine with enough power to turn a 3 blade prop of the diameter and pitch that will net the output that you are needing. The more blades on the prop, the less thrust it will provide compared to a 2 blade prop. I cannot tell you why, but I know it is true. I fly a 15x11 4 blade on a 1.60DZ that power an 11 pound plane. It only works because the engine has power to spare to turn the prop.

the plane should be in the 15-16lb range and the engine is a fpe 3.2 do you think i have power to burn? rather not jack up the front

Should not be a problem. Just "underprop" slightly and you will have adequate thrust (21" x 10 or 11). I have had great success with the Mejzlik, Biela , and my current favorite the Bolly (lighter, spin up is faster).
Unfortunately its not cheap to experiment with three bladers..