Its just a thought, but if the 3 of you have been working on your plot for several months and no- one has said anything, why would it change? I can see why the council would put a clause in there like that just to cover themselves, but do they really police it
In theory you are breaking the rules, but would the council go out of their way to cause you grief over it if they are getting their rent and the plot is productive?
People pretty much do as they please on our site. Some share and some have friends and family to help. Unless the plot is officially split, only 1 of them is the tenant, but they work the rest out between themselves and no-one says a thing.
The allotment officer at the council does do periodic inspections, issues weedy notices and the like, but vetting people as to whether they are the allotment holder, a friend or just a visitor - nope. We are lucky to see him once a year tbh.
The danger with asking too many questions of the council is that you get some jobsworth who follows the rules to the letter and makes an issue of it. If your council are anything like ours, they are pretty inefficient at most things and won't even notice
Decide among yourselves who is the official tenant for the council paperwork, stress not and garden on