I think this is the best time to potty train a kid. Send him outside with just shorts on. Peeing down his leg should be a deterrant enough.
We put a potty seat on our deck with a roll of TP. If there is an "accident" Just hose him and the deck down. My kids quickly got the idea that taking a break to sit was better than the alternative.
For me, I think underpants are just another thing to have to wash (when you're at home dealing with the bulk of "accidents") and training pants/pull ups just teach kids it's OK to still pee in their pants.
If you do have to go out, do so immediately after he pees. And put a "pee pad" under him in his car seat, bring extra clothes and you can always hide a pee pad in the shopping cart under a blanket too. (You can buy them in the adult incontenience section.)
If he says he doesn't like it, tell him not to pee pee in his pants or tell you if he needs to go to the bathroom. And that until he keeps his pants dry you HAVE TO protect the seats.
Like anything, it's a process. Most drs will tell you that physically, 3 yr olds should be very capable (physically) of holding their pee and using a toilet. Unfortunately we all use diapers long beyond when they are capable, which then leaves us "stuck" with the battle of wills.
We've all been there.
Best wishes.