An Oyster card is two things:
- First, a stored-value card that you can load money onto and pay for travel trip for trip (Pay As You Go),
- Second, an electronic container for Travelcard, which offer unlimited travel within several zones in a particular range of days.
It sounds like you have bought an Oyster card that is preloaded with 7-day travelcard for zones 1-3.
Supposedly the PAYG fares play well together with the Travelcard so you only pay for the part of your journey that is not covered by the Travelcard zones. So you can just load some PAYG money onto the Oyster card (there are machines for this at all stations) and then touch in and out normally.
How much it will cost depends on how you travel.
If you're going from Woolwich Dockyard station, that is in zone 3 and covered by your Travelcard.
If you're taking the Southeastern or Thameslink trains from Woolwhich Arsenal station towards London Bridge etc., the peak fare to Woolwich Dockyard (which is the first zone-3 station) is 2.60 pounds. Off-peak is only 2.20 pounds.
If you're taking the Docklands Light Railway from Woolwich Arsenal, that is only 1.70 pounds or 1.50 off-peak.
What I would do in your situation is load 10 pounds onto the card and check at one of the machines on the second day how much your travel so far has cost you (you may have been lucky and some of them counted as off-peak) and whether you need to add more money (or detour via the DLR) to cover for the last journey.
Alternatively take a bus to a station in zone 3. Every travelcard, no matter which zones, is valid for buses in all of Greater London.