You originally said that the other 2 were running RSTP. If they're running legacy STP, you need to remember that they'll form their own STP "group," with only one active link to the RSTP "group." My recommendation would be to connect these to the RST...

You generally need to run the same version of spanning tree on all of your network devices. The notable exception being that if you have RSTP and STP, RSTP will see an STP region as a single "device" and give it exactly one functional connection to t...