Hi..as per my knowledge RPi 3 can, in theory, draw up to 3A from 5VDC, this is pushing the limit of the 24VAC power supply: 3A*5VDC = 15Watt, this means just supplying power to RPi 3 may exceed the power rating of the 24VAC adapter. SO I think it’s better to power RPi 3 separately with its own USB adapter.