I’ve never bothered to try not that I can remember anyway, but is it possible to do TDM 3 man squads and spawn in warthogs, I honestly don’t know
If you can use forge, definitely. You just need set three initial spawn locations for each team (Red, Blue and Green) with a Warthog next to each spawn.
Also put a soft kill zone over the entire map which does not apply to vehicles, forcing players into a Warthog.
There was a ‘World of Tanks’ game mode in Halo 5 for a while. If you can find it (using the file browser), you can basically swap out the tanks with Warthogs and it’s all set up.
Google some tutorials and play with Forge, it’s daunting but easy to get the hang of.