Remember the acronym SOHCAHTOA.
(sine = opposite/hypotenuse, cos = adjacent/hypotenuse, tan = opposite/adjacent)
As previously described, the smallest angle in a 3-4-5 triangle will be between the side that is 4 units long, and the side that is 5 units long. If you draw the triangle, you'll see that the opposite side of that angle is 3 units long. Therefore:
sine = opposite/hypotenuse = 3/5 = 0.60