If people were paying attention, then 3 would be sufficient.
At 30mph and spaced apart I may agree with you but on a motorway at high speed 3 indicator flashes could easily be missed as you were scanning your rear view mirror and side mirrors before focusing on whats ahead again. My son in law was involved in an accident the other week where a police car rear ended him at 100mph. They hit him so hard it shunted his dashcam speed up from 58mph to 73mph before rapidly dropping down to zero as he scraped along the central barrier. They said he hadn't indicated, their dashcam showed he indicated more than 3 times.