How do you calculate the no-load current of a three phase induction motor?

All Transformers, single phase or three phase have a "no-load current" rating. That is simply the nameplate rating of the Xformer in VA, (volt-amps) or KVA (1000Volt-amps) for larger Xformers. Simply divide the nameplate rating in VA or KVA by the supply voltage and you'll get the Xformers potential maximum output in ampheres.