I want to know, how can I get mesh’s faces/vertices from Loader3D or ObjectContainer3D in broomstick ?
I just want to use them for ray intersection (method: getRayToTriangleIntersection)

If you have more than one mesh in the loader, or more than one sub geometry in the mesh, you need to loop though them.

vertexData length equals to 3 (x, y, z) or n*3 ?

Quite right, the vertexData is in triplets; x,y,z
There is also an indexData if you need faces (every 3 indices represent a face),
vertexNormalData, which gives the normal for each vertex,
and UVdata, which is in normalised u,v pairs, where there is one pair, for every triplet in vertexData.