For the TC3, if you use 64 pitch gears and an 80 tooth spur (or smaller) you should be able to use up to a 47 pinion (80/47*2.5= 4.255 FDR) without having to dremel the chassis under the pinion gear. Personally I've had good results running an 84 spur and up to a 50 tooth pinion (4.2 FDR - max allowed), but you will have to dremel the chassis.
The 69/31*2.5 = 5.56 FDR using 48 pitch is more appropriate for running 4-cell/27t motors. If you are going to run 21.5/lipo you at least want to start out at 4.5 FDR depending on your track rules (some limit max FDR), track size, and setup.
The 69 tooth 48 pitch gear is equivalent to a 92 tooth 64 pitch gear. (
http://www.robinsonracing.com/tips.html). If you already have 48 pitch pinions, you may want to base your spur purchase on that. Robinson does sell a 60 tooth 48 pitch spur. 60/35*2.5=4.28 FDR