First WW3 is not going to happen in less than 10 years (if it happen some day !).
I am not so sure about it. Almost all the major candidates (Bush, Clinton, Rubio, Christie.etc) for the POTUS 2016 elections support invading both Russia and Iran. Only Trump is against this.
I'd prefer to join Iran and Russia against the evil USA, but if those two declare war to EU (0,0001% probability to happen), I'd have no problem to join USA against those countries that I love (maybe except China).
Most of the European Union countries are part of the NATO. So if the US invades Russia, then these nations will also take part in the invasion. And Russia will retaliate against them by firing its SS-18 Satan nuclear missiles.
The manpower could compensated by picking men in Africa.
They will make good cannon fodder for the Russians.