There have been several threads on this but my experience is that if the tubes are replaced on the slots 3 weeks isn't too long. Someone else who has claimed similar or maybe greater times than 3 weeks recommends cling film over the top of the Nova as well.

There is a ratio of replenishment for RA4 and I'd err on the safe side and certainly replenish even if you only did a couple of sheets.

Do this then try one sheets through the "old" dev and see if there is any noticeable deterioration. It'll cost you one sheet at most and if it is fine, save you a lot over a period.

No point in throwing away dev if it is useable.