The allowances are published on the website.

OW Saphire gets the fee for the first two bags waived. The fee for the third bag is $150.

However, even though you are not connecting at LAX, but rather transferring across tickets, BA may be willing to check your bags to SMF and, if so, may not collect the $150.

There is nothing you can or should do in advance about this. Simply ask to have your bags tagged to SMF. You will claim them at BOS, cart them through CBP and then drop them (but not recheck them) with AA post-Customs.

If BA seeks to collect the $150, you must pay.