Is Koh Samui, Koh Phangan and Koh Tao too much for one trip?
All three can work on a longer trip, but do not pretend the transfers are invisible. One or two Gulf bases will usually feel more like a beach holiday than a ferry timetable.
Answer
Samui, Phangan and Tao each offer a different version of the Gulf: bigger-resort convenience, party-and-cove energy, and diving-focused island time. The problem is not that the trio is impossible. The problem is doing it too quickly and turning every third day into packing, waiting, boating and finding the next hotel. If you have two weeks or more for the beach section, the route can breathe. If you have only a handful of nights, choose the island that matches the trip: comfort and services, nightlife and beaches, or diving and a smaller-island feel. Thailand rewards movement, but only after you have paid for it with time.