1 union 3 unions ... whatever.... it would make no difference. There comes a point when the staff will be paid according to how difficult it is to replace them.
We have just passed that point.
Good luck to the ATSAs and engineers. If they negotiate well they will probably end up a system similar to the one which pays very different rates to ATCO 3s in towers and ATCO 2s in LACC. A b***dy painful negotiation that was for ATCOs. Airport ATSAs will get little and ATSA 4s at Swanwick will get a lot because they closed big bits of airspace.
Engineers on vital projects should get enough to buy their co-operation. Others will not. Does an ATCE 4 get the same all round the country ? If so then differentials will appear.
The ATCOs flexed just a little muscle and increased their worth in relative terms. They will do it again when management look for productivity increases.
Now the others should do the same to get fair reward. Only you can judge what is fair for your job in your part of the world.