Why only two atoms share an electron and not three?
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.