You can add the ethernet port and both wireless AP interfaces to the same bridge. I have a few APs setup that way.
The original definition of the term bridge was only 2 ports. But a standard ethernet switch is technically a mulitport bridge. In Mikrotik RouterOS you can add multiple ports to a bridge. I have used rb150 mini routers as a switch with all the ports added to the same bridge. I also have a few rb133 and rb532 routerboards with two or more wireless cards in AP bridge mode and each wlan interface in the same bridge as an ethernet port.
You are correct in thinking a port should not belong to more than one bridge. Even if it is possible I would avoid that configuration.
I hope this answers your question.