Three times in 18 days is way too often to entice her imo. She might get more stressed being introduced prey she doesn't care for that often than be in tune to her hunger (again imo).
Four feet for a year-old female is very small and I know you've brought this up before and have been told that but with her current size in mind and her history of not eating I would take a more drastic measure. In fact it almost seems that there is something not right about her environment or her prey. Burms will not willingly stunt themselves, given proper conditions. Here are a couple of thoughts:
1) At this point I think it is reasonable to try live. When feeding live you should stay and observe- if in 5 minutes she hasn't gone for the prey, take it out. Wait two weeks and try again. If she were mine I would be more worried about her gaining at this point than switching her to thawed or p/k. Also if she strikes once or more but doesn't attempt to constrict, she is striking defensively. Take out the prey - wait 2 weeks and try again. Her cage should be large enough so that she can get away from the live prey entirely if she wants to. Feeding live small to medium rats is a safe alternative as long as you observe and take out the prey in five minutes if not eaten. All of my snakes have been started on live and given a good start before being switched to p/k and many other herpers are aware of the value of giving a good start to their constrictor. Injuries and fatalities occur when herpers walk away from the cage and/or forget about the live rat. I had a boa constrictor several years ago that would not give in to any method to entice her to eat thawed or p/k rats. It was live that got her to eat and she went on to eventually switch all the way to p/k rabbits. Sometimes that she eat is more iportant than anything else.
2) Try chicks. Burms are suckers for chicks. Find a source for live chicks and p/k and offer- again on a 2 weeks schedule unless she takes to them. Give her all the chicks she wants. Again, my concern, if she were mine, would be to get her to gain before I tried to switch her to anything that others considered more appropriate. I have several giant constrictors that take full grown chickens and rabbits and I've found that chickens are a great nutritional benefit when rotated along with other prey. If you can't find live chicks or if you don't want to pre-kill them yourself, find a source that offers vacuum-sealed chicks- don't get chicks frozen in baggies.
3) In case you have not thought of these: make sure her cage is in a quiet -out of the way- place where she does not get stressed by lots of movement,lights, and sound vibrations.
4) You could always send her to me 
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Marcia Pimentel
Tango River Reptiles
GiantFeeders