Operation Unthinkable happens, one way or another - probably Soviet and WAlly troops firing on each other near Berlin and neither side backs down due to a comedy of errors.
Soviets push further into Germany, America goes 'Russia stabbed us in the back' and fully mobilizes. WAllies push slowly forward through Germany, recruiting German soldiers, maybe even ex-Wehrmacht units. When WAllies are pushing through Poland and obviously going to win (Russia having lost a army and a couple of cities to Abombs), Finland declares war on Russia.
While not much help, they did join in, and no one else wants the land, so when the USSR is carved into little pieces (e.g. all the SSRs become independent, with e.g. Ukraine reaching to Georgia), Finland gets a chunk for its help.