Odd , Ive never heard of the .15 RX motor , until now. My guess , its not worth the money. I saw it costed like $200 dollars.
I would cancel the order or return it if you can. What you should get is the os .12 TZ motor , thats os newest motor and its quite frankly very powerful and the same price if not less at some places. Youve definently got a slow motor too cause its saying something like 25,000 rpm. I bet a CV-R can beat that thing out and thats as well a very old motor. The TZ came out last year , costs the same and will have nearly 40,000rpm and double the power. Its the fastest .12 from os, hands down.
As for the tc3 , its an older car. I would have chosen something else for sure. If your on a budget the 2004 r40 can be found for cheap , its a team kit of sorts and has a lot of the option r40 users want. $250 plus shipping.
Otherwise I have a feeling your not going to be happy with either of those. No worries though , things can be returned.
Like everyone says the car not only has diff trouble , but its just plain an older design and granted you paid about as much for a current car and engine , you might as well get something thats worthy for todays racing.
But yeah at least get the TZ , and I would seriously return that tc3 too.