We consider the scalar integral associated to the 3-loop sunrise graph
with a massless line, two massive lines of equal mass $M$, a fourth line of
mass equal to $Mx$, and the external invariant timelike and equal to the
square of the fourth mass. We write the differential equation in $x$
satisfied by the integral, expand it in the continuous dimension $d$
around $d=4$ and solve the system of the resulting chained differential
equations in closed analytic form, expressing the solutions in terms of
Harmonic Polylogarithms. As a byproduct, we give the limiting values of
the coefficients of the $(d-4)$ expansion at $x=1$ and $x=0$.