I don't think AA's gates at Terminals B & C can handle anything bigger than a 757. B6 can, but it only handles the daily DFW-ORD-LHR flight.
As for Terminal A, there is no question your 777 flight will arrive there. All of American's international flights arrive & depart from there.
I thought for Terminal B, it was only used for flights to certain cities such as LAX, PHX, and ORD.
For terminal A, there are some domestic narrow-body flights handled out of there, but most flights are either international(wide & narrow) or domestic(widebody).
Terminal C handles the vast bulk of AA's domestic flights. That terminal is domestic only.
If your LAS flight leaves out of terminal C, be sure to pay a visit to the admirals club, it doesn't get much better than that. Regardless of the terminal, try and get in to the club. There is an Admirals Club in every terminal that AA flies out of, and whether the Ambassadors club is still open, try and get into that in terminal B(TWA club)
There is a new club in Terminal A that I haven't been in, but they tore down the old club which was aging in my opinion.
The terminal C club isn't anything special but offers a great view of AA's gates at terminal B, not to mention the proximity as it is basically in the middle.
Hope this helps.
Give me a break, I created this username when I was a kid...