Is it possible for Cytosine and Hypoxantine to make three hydrogen bonds?
I strongly believe that it won't be possible for these compounds to form three hydrogen bonds.

Revisiting the basic definition of a hydrogen bond:


  A hydrogen bond is an electrostatic attraction between two polar
  groups that occurs when a hydrogen (H) atom covalently bound to a
  highly electronegative atom such as nitrogen (N), oxygen (O), or
  fluorine (F) experiences the electrostatic field of another highly
  electronegative atom nearby.


Such electronegative atoms as oxygen  and nitrogen with free lone pairs are potential hydrogen bond acceptors.

Hydrogen atoms attached to very electronegative atoms like O and N have strong partial positive charge and these are potential hydrogen bond donors

These two crucial requirements for hydrogen bond formation, have thus been summarised:


  In order for a hydrogen bond to occur there must be both a hydrogen
  donor and an acceptor present. The donor in a hydrogen bond is the
  atom to which the hydrogen atom participating in the hydrogen bond is
  covalently bonded, and is usually a strongly electronegative atom such
  as N,O, or F. The hydrogen acceptor is the neighboring electronegative
  ion or molecule, and must posses a lone electron pair in order to form
  a hydrogen bond


Here is an impression of how these two would bond:



Note the "X" illustrating the missing requirement(s) to form a 3rd H-bond


missing potential donor group (electronegative atom) just C present.




Acknowledgements


Hydrogen Bonding
Voet &amp; Voet Biochemistry: Genetic code


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