I use 12oz gloves and 3.5m wraps but everytime my knuckles turn blue and painfull for bout 2-3 weeks, help me?

Either you need to get a new pair of boxing gloves or better wraps for your hands. If you are using boxing gloves for the bag then you need to get bag gloves. Sounds like the padding on the boxing gloves is wearing out with the sweat that has been in them over the years of usage.