Splay legs are *usually* apparent at hatch.. not 3 weeks out.
I've not heard of anyone hobbling a 3 week old chick and seeing improvement, before you.
My *guess* would be the vitamins are working. Your feed routine didn't sound extremely off to really cause a deficiency.. BUT some individuals...
Welcome to BYC! :frow
Please describe feed routine including treats and supplements.
Are you able to post pics or video?
Also, please describe your brooder set up.. lying down with wings spread *might* be an indicator that it's too warm. More details may help discern if there is any concern. :fl