The important part of balance is the spacing between firing events. For a V12 made out of two I6s, you want a bank angle of 60 degrees between them (720deg / 12 = 60deg). Or you can use a multiple of 60, so you could construct a V12 with a 120deg bank angle.
Just like for a V6, 720deg / 6 = 120deg, but by changing the firing order around you can also set the bank angle at 60deg without winding up with an odd-firing engine or split crank pins or whatever.
So if you're making a V18 with three I6s, then you naturally want 40deg between each bank (720deg / 18 = 40deg) and that looks like about 40deg even between each bank. Sometimes it's slightly off for whatever reason without making a big difference to balance, like maybe there's a funny way to get all the conrods on the "same" crank pin, or maybe it's just like how horizontally opposed engines often have the banks tipped up a few degrees to promote oil draining black into the block vs. being perfectly 180deg apart.