Should I get three tens or one 15? (Sundown audio)?

Well, for starters, "tight punch" is not determined by sub size. It's determined by sub and more importantly ENCLOSURE quality. The best SQ 10" sub in the world can sound sloppy in a bad box just as a $100 run of the mill sub can sound great in a custom designed and quality built box.

The X10 and 15 have the same xmax so their low frequency capability should be pretty comparable, but the 15 would obviously be much louder.
When you stack up 3 10's against a 15 the 10's would have about 25% more cone area - but that only amounts to a single dB of loudness.

The other issue is power. The 10 and 15 are both 1500w subs. So to adequately power 3 10's you'd need 3000-4500w rms vs 1500 for one 15 and the 15 would be almost as loud.

The 15 is the way to go IMO.

One sub will be loud enough to satisfy anybody. Unless you are a basshead and you just gotta have 4 15's in your vehicle.

If you want the 3 10's go for it. If you want the 1 15 go for it. I can't decide for you. That's like me deciding if you should get the Camaro or the Mustang.

Three 10's is overkill. One is more than enough to make your ears bleed. and has the same power rating as the 15. Besides that, your alternator probably can't handle an amplifier that can adequately power three 10" Sundowns. Get a single 10 or a single 15 and choose according to which sound you prefer. Personally I'd go with the 15 because for any given volume it requires less power because it is significantly more sensitive than the 10" sub at about 87db vs 81db.

I would get 3 tens, personally. My car has two 12's and they sound amazing. Multiple subs produces more sound and energy but it all depends on what you want. Also it depends on the kind of car you have. But if you want the most power, I think 2 12's is a good choice, but that's my opinion.