Here (refered to documentation, which will soon be published by Shelly) are commands with which you can query and set the BLU TRV via HTTP via the Gateway Gen3:
Get (first _BluTrvID_: 200):
http://GatewayIP/rpc/BluTrv.GetRemoteStatus?id=_BluTrvID_
Set (first _BluTrvID_: 200, _Temp_:4.0 - 30.0):
http://GatewayIP/rpc/BluTrv.Call?id=_BluTrvID_&method=Trv.SetTarget¶ms={"id":0,"target_C":_Temp_}
Jo_Be