Tuning my guitar but it's saying 3rd string is F when it's supposed to be G HELP PLEASE!?

It's because it's so out of tune, the tuner doesn't 'know' which string your plucking, it just tells you when your close to a note. I'd imagine your 3rd string needs tightening from f to g. Google online guitar tuner and the pitches should give you an idea when you're close.