Two ways are always reserved for the hot and cold water inlets while the others are for the water outlet(s). Therefore:
A 3-way mixer allows 2 water inlets and 1 outlet (e.g. for a shower head);
A 4-way mixer allows 2 water inlets and 2 outlets (e.g. for a shower head and a hand shower);
A 5-way mixer allows 2 water inlets and 3 outlets (e.g. for a shower head, a hand shower and a spout);