Douching is absolutely never a good idea. The vagina is a self-cleaning organ that needs no help from outside sources. Using douche upsets the natural pH balance and can significantly increase the risks of yeast infections and other issues.
It would be an even worse idea to use it so early in the postpartum period. At three weeks postpartum there's not even a guarantee that your cervix is fully closed, much less that all internal and external tearing, "skid marks" or other damage is completely healed. Flushing your uterus with what's basically smelly vinegar would be asking for trouble.