Good idea to buy a 3-day tube ticket before leaving?

I don't think there are any more 3 day travel passes. You can buy either single or 7 day paper travel passes from National Rail (not tube) stations. These can be used for the Transport For London (TfL) 2-for-1 offers. No need (or even options) to purchase these in advance. The other option is to buy an Oyster card (this is actually a payment method). You then can load Pay - As - You- Go (Paygo) cash, daily or multi-day travel 'cards', or both on the card. The Oyster is NOT eligible for the 2-4-1 offers.

To elaborate on Toni's answer, the Days Out two for one promotion is a promotion of the national railway companies, not the TfL which run the Underground. TfL Travelcards are not eligible for the Days Out promotion.

Would you be kind enough to explain how the Oyster card works. Can I get one before I go? Or, do I get it at a tube station and tell them I want it for 3 days? Thank you.

No need to buy it before you go. Simply get one at a tube station. You'll pay a deposit (I think it is about 5 GBP) and it and any unused PAYGO cash will be refunded when (if) you turn in the card at the end of your trip. You can have the attendant load it with some paygo cash (10 GBP would be plenty if you are getting a travel card loaded too) and/or travel card(s) for specified zones. You'll most likely only need zones 1 and 2 and could use the paygo cash for any travel outside those zones. Or load a zones 1-6 card and you probably won't need the paygo cash at all. You 'tap' the oyster card when you enter the turnstile at a tube station and again when exiting- just watch others and you'll figure it out. Not sure how they work on buses, but again, just watch others or ask the driver.