A Muni train or bus fare is $2.75 and a cable car is $7, valid for 90 minutes. 3-day pass is $32. You can do the math, but my feeling is that the pass will be cheaper if you are relying on public transit for all 3 days.
If you want to pay per trip, you can get a Clipper card and load money on it. That way you don't have to carry cash. I think the card costs $5 and you get a 25 cent discount per fare.