Is it possible to still have discharge after getting treated for std three times ?

Sure it is...: A discharge is the body's way of trying to wash something out. It can be an infection, but it can also be trying to soothe the inflamed and irritated pelvic organs that were suffering from an STD. It could also be you keep getting reinfected by someone who is not getting treated (despite what they tell you). I'd get follow up cultures and if negative, abstain until drainage discontinues.

Yeast?: Agree with Dr M but also if you have had several rounds of antibiotics & have discharge I’d wonder @ a yeast infection (not necessarily sexually transmitted) , especially if any itching present, or bacterial vaginosis. If you were already checked for those then may just need to give it time. Remember there is no “safe sex” except in committed marriage relationship