IMO that depends on the general strength of the name. It's true if indeed the domain is something as strong as carrepair.com. Then you should probably get all three, because time travel to 1999 is risky and you should reg as many names as possible while you're there, or should I say: while you're then. Or more properly in time travel grammar: On-when you're there-then 1999 you should willan-on have registeren many domain names
CarRepairing.com is a fairly weak name though. Leave that out. (Leaven-out)
OK, OK, sorry. Seriously:
For weaker names (hand regs), I usually only get the category defining name (or closest) plus sometimes (rarely) the corresponding singular/plural. The defining name is the default form that people use and hence has the greatest search popularity.
For hand regs, I never get anything other than the .com. There are some good .com hand regs still to be found, but they are not strong enough to justify getting various tenses/forms, just the best ones, and not strong enough to justify getting any other extension than .com.
