John, the M3 you purchased is PERFECT for a 50mm lens. When you receive it, you'll see the reason, but if you want to know now... well, it's simple: almost the entirety of the viewfinder contains the framelines for the 50mm lens. Nothing wider.
Granted, you can put a 35mm lens, but you won't have even an approximate idea of the FOV of the lens if you look through the VF. So, you have a choice:
A) Get a 35mm lens AND an external viewfinder so you know what's getting in the picture.
B) Get a nice collapsible 50mm Summicron (or any other 50mm lens for that matter, in bayonet mount).
Take care and congratulations on the purchase! 🙂 The M3 will teach you to appreciate the 50mm focal length!