Will a 3lb bucket of SI fondant cover an 8" two layer cake approximately 4" tall? Or should I get the 5lber, just to be safe?
you should be fine. I believe on their web site Satinfinefoods.com they have a chart for how much fondant to use.
I use 1.5 pounds of fondant to cover a round double layer 8" by approx 4" high. I have some left over too. I like to buy a bit extra just incase.