Actually there is no limit to the total number of boosters you can buy.

So, at the end of each of your turns you cand buy 1-3 boosters and use the units drawn to replace units or bring your total up to 15 units.

It is even possible to just use the mechs from your first 3 boosters and at the end of your turn buy 3 more boosters and put those mechs in, etc, etc, until you have 15 mechs on the board.


Paladine

@ Obstegox:
I have the rules in front of me. "BUY IN. At any point a player may choose to "buy in." This means that the player may buy 1 to 3 . . . "

At any point means they can buy 3 boostes while the BM is setting up. Then they add 3 pieces at the end of turn 1. As was also mentioned, there is no limit to the number of boosters that can be purchased in total. The only limit on boosters is the number that can be purchased at each buy in. It is easy to understand the confusion. My venue and I spent about an hour discussing issues that might arise based on the wording in the rules.

My venue is going to place small restrictions on when a buy in can take place so as not to disrupt the game.