Tube Tickets in London

Lynna,
You don't buy tube tickets any more. What you do is buy an Oyster Card. You can get this as soon as you arrive in London, at any tube station, including Heathrow. This is a pre-pay card, you charge it up with money, and use it to open the ticket barriers when you enter and exit the tube.

• It automatically charges you, based on the journey you actually took. It is 2/3 or less of the cost of buying individual tickets. For example, for a journey in central London (Zone 1) it costs £4.90 if you buy a ticket at the station, or £2.40 with an Oyster card.
• If you take enough trips to reach the cost of a daily ticket, it stops charging you and all trips after that are free. For central London (zone 1) the daily total "caps" at £6.50.
• It is also valid on buses. You can no longer buy tickets on buses from the driver.
• You can recharge it at most newsagents and small shops in London, as well as tube stations.
• It is valid on tube from Heathrow to central London, but not on the "Heathrow Express".

Details are here: https://tfl.gov.uk/fares-and-payments/oyster/what-is-oyster

How do you know how much money to put on it when you get it?

Lynna,
I would put on £20, plus there is a deposit (£5 I think) when you buy the card. Since if you are staying in central London, the maximum you pay is £6.50 per day, that would last 3 days.
Whenever you use it the display at the ticket gate says how much you have left, when it is getting too low just drop into a shop with an Oyster sticker on the door and say "Can I have £20 on this card please".

When you leave London you can get a refund of whatever is left on the card (including the deposit), or just keep it for next time. There is no time limit. My card sits in a wallet with leftover pounds for next time I go to London.

You can also add any amount to your Oyster at the ticket machines in the Tube stations themselves. I usually put my spare change on the Oyster to empty out my pockets.

Very helpful for us newbies. Thank you for taking the time to respond to the original poster's question.

If you're only staying in London for 3 days, 20 pounds should be plenty and then some.