There are lots of things to try and lots of info to be found by using the search function. Off the top of my head you could try: anole scenting (saliva), a live pinky, brained f/t, rat scenting or the head of a rat pinky. There are quite a few other tricks that can be tried as well. I have had the best success with live pinkies. 2 of 3 non-feeders I've had ate a live pinky no problems.
Three weeks isn't an exceptionally long time to go without food. Just keep an eye on him/her and see if it looks skinny or lethargic. 3-4 days between feeding attempts is probably okay, but you might try going 5-6 days. I know it's not that great a difference, but offering too often when they are refusing can cause them to form a habit of just refusing every time they are offered.