The inner hinge pins on the TC3 and TC4 (a-arms) are not the same diameter, so they are not a direct fit. The a-arm mounts also are different so they aren't a direct fit either. Bottom line is that many parts
may fit on each car, but may require some modification. Also, some Losi XXX-S parts will fit on the TC3 (steering knuckles, caster blocks, rear hub carriers)
All of the drive line parts are interchangeable
EXCEPT the spur gear assembly holders. Also the TC4 drive shaft/input cups use smaller roll pins. So if you use a TC3 shaft on a TC4, you will need to use TC3 input cups.
Many people have created their own version of a TC3.5 by using the TC4 shock towers, chassis braces, and steering knuckles (all are a direct fit onto the TC3).
Some aftermarket chassis also had the ability to use a swing steering rack (
http://www.xtremercracing.com/Produc...10040&pID=1155).
Bottom line is that I've run a stock TC3, tub TC4, and FTTC4 in VTA and was competitive with all of them. Each car has quirks (TC3 rack gets dirty, tub TC4 is a pain to adjust shims under a-arm mounts, FTTC4 is a pain to replace a spur gear/change pinion & you have to cut an ear off your servo), but all are capable of a podium finish.
Hopefully this info helps.