I have a 3 yr old beardie. I was wondering how much food are you supposed to feed them a day?

Feed them their veggies (collard greens, mustard greens, alfalfa, squash, etc ..) every day, a big salad because that's what they mostly eat when they're adults, and feed them crickets (dusted crickets) once or twice a week.

I own 2 bearded dragons.

Depends on the beardies. my female eats ravenously while my male has to be hand fed he won't eat out of the bowl. they get a salad and some worms and some peas and carrots. variety helps out a lot. also leave the food in the bowl for an hour at the most and throw away any food they haven't eaten yet. they will stop when they've had enough or if they don't touch it at all try the variety thing or misting the food or hand feeding.