"So in the 3rd Edition book, there's an interesting twist to that setup.
Defender units falling back actually fall toward the center of the board and automatically regroup upon reaching the center! Such is the time of desperation. It's really wild.
Also, Defender Reserves would come on from the center of the board, as if they're being dropped in, emerging from underground, etc."
In other words, we have to make sure Sam announces this before the start of the third round.
And Flyers:
"Yes, they'd come on from the center of the board, and yes, they'd re-enter from the center. (Picture the planes doing loop-de-loops!)"