Duration of work in the US to that requires a TN Visa

If I'm going to work in the US for 3 days no longer would I require a TN visa. Do you require a TN visa for any period of work?

Yes, even if it's 3 days you must still be legally authorized to work in the US.

Depends on the nature of the work, a lot of stuff is actually allowed as a regular B-1 visitor under NAFTA. Do a web search for "B-1 NAFTA entry" and read the regulations. You might find what you want to do is allowed.