Commuting between North Acton and Wimbledon (both 3rd zone),
and exchanging at Notting Hill Gate (zone 1 or 2?),
a Travelcard for zone 3 only or for zone 2+3 (1+2+3?) must be
purchased?

The travelcard required is based on the zones travelled through. I believe
North Acton is actually on the zone 2/3 boundary and Notting Hill Gate is on
the zone 1/2 boundary so a journey between them would be treated as all in
zone 2. Notting Hill Gate and Earls Court are both on the zone 1/2
boundary but to in travelling between them you will travel through High
Street Kensington which is zone 1. Then once you get beyond East Putney you
are in zone 3. So you need a travelcard covering zones 1+2+3.