A few years ago i took plan b a little often that i should have it that a reason iam having trouble conceiving ?i been trying for 3yrs now, no luck

No harm: Using Plan B should in no way have harmed your fertility, no matter how often you used it. Three years is a very long time for a 20 year old, see a good gynecologist or fertility doc for the help you need.

No: It should not have any effect. If you have been trying for 3 years at a young age, likely there is a significant issue and i would recommend seeing a fertility specialist sooner rather than later. That is not very common and so there is a higher chance of something severely affecting the tubes, the eggs, or possibly having no sperm.