No such thing as "merging" for these purposes.
The policy is that they don't get included MCE or bags, but many agents will do it for you. As to seat assignments, try calling AA. They are generally helpful. Same thing with bags at check-in.
Alternatively, as PLT, you ought to be able to check 2 bags. If each of the three people on your PNR check 2 bags, won't that work?