Actually I think I should clarify my post. I didn't mean you'd gear it crazy, I meant if you think the available gearing of the trans would work with the 21t on 3s. The 21t on 4s is probably roughly equal to the rpm of the 12t on 2s, so on 3s the 21t is going to be spinning a bit slower. But it should have more power to pull the higher gearing and increased weight of your Slash better than the 12t. There's also no reason you can't run the 21t on 4s depending on what you have on hand and/or are willing to buy for the project.