Set MacBook Pro/Air to disable clamshell mode when plugged in only

I have three Macs (old iMac, MacBook Air, and a MBP) and I switch between them quite frequently. I have most of my files shared via Dropbox so I can access most anything on any computer, but occasionally I'll be using the Air and need to access a file on the iMac or MBP or trice versa, so I have Screen Sharing set up with shortcuts on my desktop to easily screen share to one of the other computers. This means I have to leave the computers on/open all the time.

I'm looking for a way to set...

Set MacBook Pro/Air to disable clamshell mode when plugged in only