It has been 3 weeks since she broke her leg? The vet say what type of break? Compound- bone protruding through skin, Greenstick - bone is crackedand bent Simple -bone broken no splinters, under skin may still be aligned, Comminuted - bone ends shattered.
If been 3wks the bone is already starting too knit together amd heal. You could try a chicken sling to support her weight to see if she will straighten her leg. Search here in forums some good pictures and how tos on the sling. You can gently examine her leg, compare it to her good leg and see how the healing is progressing. I do think it may be too late to try a splint. You will be one to decide that though after you examine her to compare the bones.
If you decide to splint get tongue depressers, or stiff plastic, pad the side to her leg with gauze or cloth not too thick, tape it above and below the break to prevent movement. Check to make sure not too tight by checking circulation below the injury by lightly pinching and releasing to check for unrestricted blood flow. If remains pale it is too tight. Even in the scaly part of a chickens leg you can see blood flow this way. It is like squeezing your finger and it becoming white when you release it becomes pink again. Not much pressure is needed. She will need to have her movement restricted in the chicken sling to prevent weight and movement of injury.
This may not work as I think chickens can be healed in a month depending on the break. I would assess her by putting her in the sling and see how she reacts.
