3 printers is not the problem. But i think i heard same problem from another one. But since it is not the servers responsibility to execute it all that matters is that firmware sends an ack. If not it will never time out since server knows that M109 is slow. So van you try with ping pong enabled and check the code and see if somewhere a timeout happens. My guess is that somehwere else an ok is missing here and after M109 you have not enough free buffer to send and the M109 prevents the timeout. With ping pong the timeout should happen at the command where the ok was not send,
