Can a three phase transformer be loaded as a single phase transformer?

Yes, but the load can not exceed the rating per phase and the load must be balanced. (KVA/3 per phase)

For example: A 75 kVA 3 phase transformer can be loaded up to 25 kVA on each secondary. If you need a 30 kVA load, 10 kVA of load should be supplied from each secondary.