DX3 (or is it DX3.0) is a simple radio with some basic functions. I don't like the plastic casing quality somehow, although I've been using other FM transmitters in the same casing - they are ok. Maybe its the gray color that looks cheap
DX3R has almost all the functions you'll ever need + 10. DSM2 is great and the size/weight of the transmitter is another big advatage. Doing 60-minute main finals and practicing on VRC really kills your wrist when you have to hold a heavy transmitter (3PK etc).
Its often said to buy as good and as expensive radio gear as you can afford! Ok, if you are doing only "backyard bashing" then DX3 is ok
BTW - the new DX3S does not support DSM2, but has telemetry.