In message , ***Jos***
writes

£18.90 + £18.90 + £3 = £40.80

This works out cheaper for you than using 10 single tickets each or Oyster
card.

Thank you very much!
That's the answer where I was waiting for.

It is actually a bit more complicated than that.

This option (a 3-day off-peak travel card) cannot be used before 09.30
on Mondays to Fridays. However, it covers a wide area of London.

There is a cheaper option which can be used at any time (3-day peak-time
travel card) which can be used at any time, but it covers only zones 1
and 2 - that includes most places in Central London that tourists visit,
but not those further out (such as Greenwich or Kew). It costs £15.40.

But it can work out cheaper still to buy daily travelcards. If you only
need zones 1 and 2, and not in peak time, then 3 daily travelcards will
cost £14.70.

In all cases, a travelcard covers underground railways and surface rail
(including the Docklands Light Railway) in the specified zones, plus
buses anywhere in London - and it offers a discount on scheduled
river-boat services.
--
Paul Terry