this may sound stupid but if you can work up to 15 hours a day then if i have a poa for three hours through out the day does that mean in total i can work 18 
Simple answer = NO.
As a (vocational) driver you are goverend by more than one set of legislation i.e. tacho regs and WTD.
These run side by side and you can’t ‘break’ either of them. So, the 15hrs working day you quote is one thing but you can’t then add 3hrs of PoA. The PoA would be part of the working day.
To be ligit, you need to have an understanding of all the regs that apply to you and be able to work out a working pattern that doesn’t contradict either/any of them. That’s the bit where we become professionals!