depends what your allowance with AA is... I presume that its two bags.

You should be OK going there, but as you are staring on AA in BOS, what are they like at extra bag charges? did you ask on the AA forum?

As you are starting from NCE with BA on way back, i would say that would you probably get charged for the third bag.

As it is an outstation / handling agent, they are quite strict in charging excess and they are not always familiar with oncarriage / bags allowances

here are the proper rules:

http://www.britishairways.com/travel...c/public/en_gb