For this topology to work properly with RTG, you not only need to disable RSTP on Switch 1 ports part of RTG but also the interfaces towards Switch1 on both Switch 2 and Switch 3. Here is a link explaining the recommendation in detail:
What mriyaz mentioned is correct and just to add you have to disable XSTP on the interfaces facing to the RTG switch, even though the RTG speakers drop XSTP BPDUs , the recommendation is to disable it to avoid extra processing ( generate and drop actions) at both sides.
To configure the RTG on switch 2 and switch 3 you have to completely disable the rstp on the respective interfaces connecting towards the RTG switch 1. So you may not put those interface in any rstp instance. Hence those interfaces on switch 2 and switch 3 would never generate any bpdu towards switch 1.
However you may run the rstp on other interfaces on switch 2 and switch 3 which would work as expected.