Ah ok, so they are going in an arched opening - I guess with soldier brickwork above.
I dont get involved with upvc, so Im not sure what trims on the head are needed. In timber we would screw on a bit of 4 x 2 or similar on the head then scribe a flat board of ssy 100 x 18 to the shape. I assume internally its a square head.
In upvc, there might be a special trim you can buy for arched heads, If not just buy a suitable length of trim. You would something above the windowframe to fit it to.
If you are a competent Diyer, then fitting 3 upvc windows isnt all that hard. Some advice on here would get you doing it in the correct order. If you are happy to pay somebody, then thats probably best - if you can find a decent fitter, you want a local independant guy.....assuming he will fit windows he hasnt supplied as he isnt making anything on the windows and they may be different to his usual.
Dont forget you need fensa / certass certificate unless the project is under B/ regs.