I did a urine sample at my first midwife appt, they then asked for a 2nd sample but couldn't tell me why! they have now asked for a 3rd! what's wrong?

Probably nothing: Many people now check urines on pregnant women at each check to monitor proactively for things like signs of infection, protein or blod in urine, monitor fluid status. It is likely this is just part of your midwife's routine and that nothing is wrong at all. It's just like taking your pulse, blood pressure and other vital signs when you're pregnant.