As previously stated if all goes well then the 3rd bag is no problem to YYZ; however you should be aware of 2 factors: if you are rerouted on AA (e.g. you are late getting through customs) then AA will impose an excess baggage charge on your invol reroute, as the 3rd free bag is a QF specific benefit only valid on QF metal; for your return trip, check-in at YYZ will also charge for the 3rd bag, if you are returning with it.

Sadly Oneworld does not have a baggage benefit, so there is no way to get around this (don't think the MSC applies to FF benefits, only to baggage allowance of the fare used).