Travelcards only exist as single day version or week (or longer) versions. For a trip of less than five days, a travelcard is not the cheapest option. Using electronic payment (contactless / phone pay / oyster) is cheapest (and simplest). Example - for the days when you're travelling in central London (not to or from an airport), you'll pay no more than £7.20.
If you intend to use oyster, don't buy anything until you've arrived in London - the visitor oyster card promoted by some resellers for advance purchase costs more (because of postage costs) than buying on arrival - and the visitor version is a less flexible product.