There is no doubt that a .357 magnum JHP will perform as well or better than any .38 and as well or better than almost any other handgun round given the same shot placement.

That said a .357 will also be harder to conceal and harder to shoot than a comparable .38 and shot placement is more important than caliber. That is the reason I keep a 6" 357 under the bed but carry a 2 inch .38.

A few other random comments:
1. For a .357 you want 125g JHPs, for a .38 most experts say to use 158gr LSWCHP +P, preferably with swaged lead bullets vice cast. The .38 need a softer compund to expand well due t its lower velocity. If you hand load take a look at power pistol, that powder can give you low-end .357 velocities at 38+p pressure with a 158gr lead bullet.

2. Personally I think 3" is long for a carry gun ... some people carry full size 1911s or beretta 92s but I would go for a 2 inch revolver every time.

3. A.357 handgun will have a longer cylinder and overall length than a .38 with the same frame and barrell length. Therefore if you aren't going to carry with .357 rounds go with the .38 over the .357.