You’ve possibly got per-face vertex normals, which makes polygon edges visible. I can see those triangles even in the Blender grey model. In mesh edit mode, you can try Mesh->Shading->Smooth Faces and Mesh->Shading->Smooth Edges
Additionally, you’ll want to check and see if the model has Custom Split Normals (see screenshot below), and clear them if it does (unlikely if you made this from scratch, very likely if you imported it).
Also visible in the screenshot is the “Auto Smooth” feature, which will override the mesh’s calculated normals and generate split normals only when an edge crease exceeds a certain angle.
I also recommend using the triangulate modifier to triangulate your mesh before export, rather than letting Studio do it on import, because then you have total control.
Lastly, if a mesh has really thin triangles, their edges will sometimes be visible in game just because of uv map distortions. Verify in the UV map editor that your item is mapped sensibly too.