After 3 weeks opened it will be quite vinegary and even more unpleasant. Even when just opened, a nasty wine will not taste good in gravy. Throw it out.
Does it smell of malt vinegar?
Does it fizz when you put a tiny bit on your tongue?
No? then its ok.
For future reference - freeze wine when you know its not going to get drunk in plastic tubs then it is there when you need a little bit for a sauce etc