It depends on what you mean by your vertices normals. A normal is always relative to a surface, not a vertex.
In the context of 3D graphic rendering, normals are associated with vertices. These normals are an approximate representation of the surface curvature and are used to provide more visual information at the rasterization phase (Gouraud/Phong).
I assume you know all that. Thing is these vertices normals are usually calculated using adjacent triangles (or manually set). If you consider them alone, you don't have enough information to get the triangle's normal back.
But if you have the 3 vertices... A simple cross-product would do the work:
N = (B-A) * (C-A)
