The gun in question is a double barrel 20 gauge and says 3" magnum on the barrel. I know for a fact 2 3/4" shells have been shot through it and did fine but I'm just wondering if it messes anything up or is fine and is it safe? I know my 870 12 gauge says 2 3/4", 3", or 3 1/2" may be used on the barrel. However, this one just says 3" magnum nothing about any other sizes. IS that the max size it will take or is it the only size that should be used?

yes

yes you can shoot 2 3/4" 20 ga shells out of you 20 ga

Yes, if it says 3" then you are able to shoot shells up to 3"....so 2-3/4" will be fine.