There isn't a reg that specifically says you can't run three fans from one isolator, but there is a reg that says suitable isolation means shall be provided for maintenance. The problem with not having an isolator is that because many fans are installed in windowless bathrooms, people would be tempted to clean the fan while running (to keep the light on) or run a table lamp on an extension lead etc. The isolator allows the fan to be off for cleaning while the light is on. If your isolator is reasonably close to all the fans I would consider it acceptable to run all three from it, however as suggested by
@brianmoooore I would put an additional isolator inline with the MVHR unit located directly adjacent to it, so that it can be serviced while the other two remain active.