  The prime graph question asks whether the Gruenberg-Kegel graph of an
integral group ring $\mathbb Z G$ , i.e. the prime graph of the normalised unit
group of $\mathbb Z G$ coincides with that one of the group $G$. In this note
we prove for finite groups $G$ a reduction of the prime graph question to
almost simple groups. We apply this reduction to finite groups $G$ whose order
is divisible by at most three primes and show that the Gruenberg - Kegel graph
of such groups coincides with the prime graph of $G$.
