Point on a triangle.

I have a triangle(part of a mesh) that is created from vertex and a green point which should be snapped to that mesh(pink triangle). The main problem is that I know only X and Z of a green point.
And mesh can’t have colliders on it. How can I find Y of that green point?

P.S.
Triangles vertex can have different positions.
Green point also can move according to X and Z.
2 attachments: Top view and side view.

The question is, how to get Vectors (GreenPoint GP) Y position on the polygon’s surface that consist of 3 vertices. We know all 3 vertex positions and Vectors(GP) X and Z coordinates. The idea is to make that Vector (GP) snap to the grid when GP changes X and Z position.