I don't know why the hairdresser has said your hair will never be long. Rubbish. Everyone has a terminal length but it is a EXTREMELY rare for anyone's to be shorter than waist.
I would either let her trim 3 inches off and stick it up, oil it every day with almond oil, use a deep, moisturising conditioner once or twice a week (depending on how many times you wash it) use only your fingers and a wide tooth comb to detangle when wet and try and grow those 3 inches back within the next 6-8 months.
OR (what I would do) Put aside a couple of hours at the weekend, section my hair, go through each section under natural light (by a window in daylight) and cut off every split end/ breakage I see. Once I'd gone through all the sections, I'd then trim myself about half an inch to an inch off the ends. Part your hair at the back and bring both forward over your shoulders and do it that way, or get a friend/ DP to trim half an inch off your hemline from the back.
I would then wash and deep condition it, oil it and I guarantee your hair will feel better.
I would then, over the next few months S&D (search and destroy = going through and snipping off all of the split ends/ breakages individually) to gradually trim out the damage in your hair so that you can still continue to grow it.
No way would I let a hairdresser cut 3 inches off if I wanted to grow it.
Oh and when you have it highlighted, tell them to only do the roots and not the lengths/ ends.