What is 3MF?
Where 3D printing went once STL's limits got too obvious. It stores each vertex once and refers back to it, and carries colour, materials and print settings that an STL has nowhere to put. Being a zip of XML, a 3MF of the same model usually lands at a fraction of the size.
What is OFF?
Object File Format, and about as plain as a mesh can get: a count of vertices and faces, then the vertices and faces. It comes out of academic geometry work, where nobody wanted to parse anything more complicated to test an algorithm.