Hi
I can't see any issue with this. The system would first check in ST1 and the bin maintained in the MMR. If there was insufficient capacity, it would then search ST2 for an empty bin. If no bin existed in ST2 the system would look for the next Stype in the search sequence and place the stock in ST3.
I haven't tested this though, suggest you set it up and test for yourself.
Cheers
A