Why only two atoms share an electron and not three? In a covalent bond between two atoms, an electron from one of the either atom is shared by overlapping of their orbitals.

So,
Why can't three atoms share an electron and overlap their orbitals?
The normal case for a covalent bond is indeed a 2-centre 2-electron bond.

There are however cases, where 3 centres (= atoms) either share 2 or 4 electrons.


In organic chemistry, the most famous case for an electron-deficient 3-centre 2-electron bond is the 2-norbornyl cation.
In inorganic chemistry, boranes with their banana bonds are the role models.


Hypervalent molecules with 3-centre 4-electron bonds, like $\ce{SF6}$ or the bifluoride anion $\ce{HF2-}$ have been discussed here too.