Can someone recommend a online multiplayer murder mystery game preferably in a 3D world?

One perfect example is The Ship, basically its exactly what you asked for, an open 3D multilayer murder mystery. Available for 20 dollars on Steam.
Also check out Spy Party, not a MURDER mystery but it is a 1v1 game in which one person plays a spy among a bunch of other NPC characters and one person plays as the sniper trying to figure out who the spy is, IT is currently in paid beta for 15 dollars.