Edinburgh

I am traveling to London from March 10th through the 12th and have decided to purchase a Travel card for zones 1-3. Is this possible? Also, can I buy one once I land in Heathrow airport? I found them online, but don’t know if I have enough time to wait for it in the mail. Also, do I need a passport, or something similar, photo for the Travel card?

I am visiting London for 3 days, and I want to know the best sights to see. How do I travel around London, by bus? Where would I buy the passes and such? Suggestions please?

I am going to London for 7 days and I’d like to know what is the CHEAPEST way to travel to tourist attractions everyday there for 7 days by tube.

Thanks!