Hi Kevin,
Yes, that looks like a good plan, particularly if you plan to make other trips at weekends etc. Travel beyond zone 3 would come from PAYG as you say, and yes you can add a railcard, or get the gold card discount applied to reduce those off-peak fares.
You might also want to check out a cheaper option. Clapham High Street – Denmark Hill – Lewisham – Woolwich Dockyard only takes 45-55 minutes but means you only need zones 2-3 for your travelcard.