My optometrist said my eyesight is 3 bars away from 20/20. what does that mean?

Probably 20/50: On most standard visual charts, three lines below the 20/20 line is 20/50. I am guessing he meant for that eye that he could not improve the vision to better than 20/50. This needs evaluation because at your age, you should be able to see 20/20 unless there is some eye disease. You should see an ophthalmologist for this and see if treatment is needed.