Not an M3, bud. The contour of the side silhouette alone proves that, since the M3 series had a sharp rather than smoothly tapered outline. The main gun is depressed, which may be what's throwing you off...but if you look carefully, it's angled from the center of the tank to the right. The M3 75mm gun would need to be angled to the left to look like it was at the center of the tank, since the sponson was all the way to the right side of the bow. Lastly, the turret is centered, but we can't see the 37mm gun, which should be visible from the angle.