The boiler is a 28kw and it is a range cooker. The total pipe being used it 12 meters with 3 bends.
That it is a range cooker is almost no help at all, the amount of gas it requires, IE the total possible KW output is what needs to be known. Saying "it's a range cooker" is like saying "it's a big boiler", it doesn't give the right information.
I would also doubt that there are only three bends on the run. It is technically possible to achieve this installation with only three bends, but it's highly unlikely a situation would arise where it is
actually possible. You're not just counting the bends under the floor, but the entire installation from gas meter to outlets.
I'm also intrigued as to quite how your plumber is going to achieve running the installation in waste pipe. I see two problems - firstly, how does he expect to get support clips on the gas pipe if it's inside another pipe? Secondly, how is he going to cover the tee for the first appliance? He can't solder first then put a plastic tee over, because it just won't go, and he can't put the tee in first because it'll melt when he tries to solder.