The beam would need to be at least three 2x12's.

Why not span the 12-9 dimension and avoid a beam? 2x8 joists would be at their limit and are likely to feel bouncy. I would use 2x10's spanning 12-9.

2X10 16" on center?

yes 16" o.c.

Here is the "Joist Span Table" from the IRC 2003 building code for 40lb live load, no notching or boring:

2x8: 12" o.c. 14'2"
2x8: 16" o.c. 12'10"
2x10: 12"o.c. 18'
2x10: 16"o.c. 16'5"

You can see that 2x8, 16"o.c. are at their limit but would be safe but "bouncy". 12" on center would stiffen it up more but would still provide some very subtle flex. Floor decking of 1" plywood would stiffen it to where you would not notice and would be enough to lay tile safely. The best, no worry would be bigger 2x10 on 16" centers. Make sure you get a minimum 3" inches of bearing if the joist ends are going to sit on concrete or 1 1/2 inches on wood.

Thanks for the input. I am going with 2X10, 16" oc with joist hangers, hung on 2X10 stringers.