I think 3" would be acceptable, especially if you need the bench to be portable.

If your benchtop is the same length as Will Myers', and you are both using SYP, then yours would weigh about the same as his if you make it 3" thick at 16" wide. If you make it 4" thick @ 16"w, then it would be about a third heavier than his.

Yellow pine is rather dense and stiff for a softwood.