Male in twenties, estimated gfr 80 ml/min/1.73m2, and anion gap 3. what can cause this? kidney issues or diseases? anything i can do to improve kidney function?

If you are a big young man, your BSA is likely much higher than 1.73 m2 so adjusted GFR can be above 90 cc/min, i.e. normal. You can google BSA calculation for calculating your BSA. Give example: if BSA is 2.0 then you eGFR will be 80 x 2 divided by 1.73 = 92.5 cc/min which is normal. There is nothing to concern about anion gap of 3 if bicarbonate is normal. We only concern about high anion gap.