I actually collected Spawn and GR comics when I was younger, and I have lots of respect for Hellboy, but I think a all out 3 way fight, Hellboy and Spawn would probably just quit, not die, not lose, just simply quit and say, 'screw this' this is pointless. Ghost Rider (both of them Gen II & III) would probably keep fighting until the end of time if you let them.
There are 2 Ghost Riders that are well known. Zarathos the Demon, and the Spirit of Vengeance, which is not a Demon nor an Angel, but simple is alive to exact vengeance through whatever means is necessary, even if it means fighting until the end of time. He does wear down after heavy hits but doesnt mean he will go down easy, it takes a lot to take down GR, he's been blown apart, vaporized, crushed, even hit with his own hellfire, but that hasn't stopped him yet.
Hellboy has some great agility and punching power, Spawn has some impressive powers of his own which even today have not really been capped yet. Ghost Rider's powers have also not been overly explained to their limits, because the character itself doesn't even understand them.
All three kick ass, but the fight is pointless and only GR is senseless and mindless to fight until the end.