I quit smoking 3 months ago. i still find it hard to breath. i get winded walking up stairs. should i not be improving by now?

May not: And may never either. When you stop smoking with already a bad lung, the lung does not necessarily improve. Stopping smoking will bring the decline of lung function parallel to a nonsmoker in time but because now it is starting at a lower level of function, always will be lower. The rate of decline or worsening of lung function is what is helped by smoking cessation. The damage not repairable.