How to Combine Objects Together Without Making Them Glitch Through Each other?

I am creating some stairs for the scene I’m in. I have 3 cubes that are stretched out and put together. All the measurements are even but when I combine them using an empty game object, they seem to be glitching through each other since they’re positions are intersecting in exactly the same spot. I need to know how to combine these objects to make a single shape so that in won’t “glitch itself”.

whenever you have two faces at the exact same position in a 3D render, you might see this kind of behaviour called z-fighting.
Z-fighting - Wikipedia.

When creating 3D scenes, you must avoid having multiple faces at the exact same position. Ether build your stairs from scratch in a 3D modeling software from one mesh making sure no faces overlap or try to find a way to design your stairs from Unity primitives without overlaping geometries.

Good luck!