After 3 weeks in the fridge you have lost viability. How much I don't know. In a sense you are now pitching a smaller starter than your original.
I'm sure it will ferment your beer. If your original starter was optimum for your beer you are now underpitching. So it would be best to refresh the starter.
Since you are brewing later today you don't have many options. You could add a little wort and let it go as long as possible. If you have a LHBS you could get another vial. Or just pitch what you have.