Never heard of an M3-P.
OTOH, an MP-3 is nothing more than a retro-looking M6 with 3 sets of frameline (35-50-90), just like the good old M2. It is a "modern" MP, not the original MP that is highly sought after collectible.
So in short, an MP-3 is more or less a metered-version of M2, but...