What are some tips to get the "blow up 3 enemies" achievements?

I got both of these during story missions.
The car one I got in the first mission where you have the cars (shrouded bunker), there a part where you enter a porch overlooking a room, with a short spiral staircase on the right, enemies tend to bottleneck just after the stairs, you can hide behind the porch's rail and send cars down the stairs, with some luck you'd get to the enemies before they blow up the cars and blow up 3-5 of them. I managed to do this on Nightmare after a few tries.
I got the arrow one during the prison break mission just after releasing the prisoner, there was a a part when he's busy hacking and you have to fend off enemies, a few guards grouped near the left side of this area and I shot one with an arrow and it got the job done. A better opportunity was in Dead City Reverse, there was a part where you watch a few soldiers fight off mutants, while they were busy with the mutants, I used my arrows and blew them all up.
If you're past these missions, I recommend doing this either on Bash TV, sewer missions (DLC) or later in the game in the distillery missions, there will be parts where you're on a ledge and multiple mutants will climb over, I think this will be a good place to try the trophies although I didn't get them there myself.

I got the Three Birds, One Bomb Car achievement after having the RC car upgraded, and got the achievement itself during a round of Mutant Bash TV. In the first room, there are a ton of mutants running around and none of them have guns on them. Just wait for three to get three close to/around you, drop and RC car and explode it. If your health is high enough, you should be fine and the achievement should pop.

Although I didn't get Hat Trick in this Mutant Bash TV (and can't remember where I did), it may be a good place to get it if you can get a swarm of mutants to track you and hit one with a mind control bolt.

As I recall, I got the bomb car one in the Shrouded stronghold. Lots of times when enemies take cover they are huddled together in the limited space, you can try to boost the car right into them.

I think what I did was to sneak up on a group of them, then I had the car just zip in full speed and KABOOM, I got it.

As for the Hat Trick, I did that one in the prison break out. What I did was in the part where you open the jail cells and let the mutants out I waited until they were attacking the guards, then I shot a guard with a mind control bolt. I moved him closer to the other guard then detonated him, taking out most of the mutant in the room as well.