Murray lawn tractor has been sitting for 3 yrs, cleaned carb and tank replaced fuel pump no run

You need to figure out whether the problem is gas or spark. Try pulling the spark plug to see if it is clean and dry. If it is put it back in and try spraying some carburetor cleaner into the carb and see if it runs on that. If it does then it is still a gas problem. If it doesn't then you ned to figure out if it's getting spark.