The reason why its labeled TR3 disconnect relay is because it is the 3rd Relay influenced by the position of Transfer Bus switch.
ie in Transfer bus switch in Auto causes TR3 relay to close (except on glideslope capture) also arms the automatic switching of the 2 transfer relays that allow the transfer buses to be powered by the opposite gen bus in cause of gen failure.
Transfer Bus switch off doesn't allow automatic switching of transfer relays 1 & 2 and also opens the TR3 disconnect relay (irrespective of other conditions)
That's my logic behind the labelling anyway