If Jesus died for our sins and took the punishment that we deserved, and if our punishment would have been eternal separation from God, then why was Jesus only in the grave for three days?

The sum total of sin isn't proportional to days in the tomb, as though Hades was a punishment. Jesus experienced the wrath of God on the cross, not after his death.

As for eternal separation, if that means "infinite," this fellow might have a good point. I don't actually see it that way, and I try to address this in the chapters on hell and judgment in my book Heaven & Hell. One reason I reject the traditional view is the imbalance—the intrinsic injustice—of infinite penalty for sub-infinite wrongdoing.