Regardless eith only 3kw of power he can easily get away with 2 18s and save quite a bit of money over 4 12s and get very similar results. 4kw and up is where I would lean more towards the 12s.. has nothing to do with "motor force", it has to do with more coils having better heat dissipation, but most big boy subs, especially 4inch coils will be just fine on 1.5k. That's also why bass race cars use more subs, they have to move less overall vs a setup with less cone area and they don't get as hot as setups with less actual coil area. So lots of cone area and many coils work the best.. for daily budget pounding you can be in between and really just need to balance space, budget, quality concerns, and power available to find what's best..
I'm conservative when it comes to hoe much power to put on subs usually.. many cat audio subs can take a lot of power, but its because they are god awful at actually using the power to make sound.. super stiff suspensions and bug heavy coils.. I try to use subs that aren't like that, and use effecient boxes. On a 3inch coil I would not go over 1.5k.. sure some can handle up to 2500, but they tradeoffs they made to alloe it aren't worth it to Me.. if you want a musical sub to handle 2500 watts, it needs to be a 4inch coil.