Amount = A = $50(1 + 0.03/12)^(12*m), where m is the # of months. Note that A= $50 at the beginning (when m=0). Note that I am assuming that "3%" is ANNUAL interest.
If, on the other hand, "3%" is monthly (very unlikely!!), then:
Amount = A = $50(1 + 0.03)^(m)