First you have to understand what these terms means to the ones who wrote about them and get the gist of what is said.
What is the setting? To whom was Jesus referring?.....and what is "eternal punishment"? Is the "fire" literal?
This is Jesus at the judgment, sentencing the "goats" to the same destiny as satan and his angels. So, what is to happen to satan and the angels who followed him into rebellion?
Revelation 20:1-3 answers....
"And I saw an angel coming down out of heaven with the key of the abyss and a great chain in his hand. 2 He seized the dragon, the original serpent, who is the Devil and Satan, and bound him for 1,000 years. 3 And he hurled him into the abyss and shut it and sealed it over him, so that he would not mislead the nations anymore until the 1,000 years were ended. After this he must be released for a little while."
So the time comes for Jesus to judge mankind and satan is bound and thrown into a confined state for 1,000 years along with his hordes. Those who fail the judgment (the goats) are not joining satan in his prison, but are actually eliminated from existence as those who have proven unworthy of the gift of life.
Their eternal punishment is annihilation in "the lake of fire", which is the final sentence for the devil as well. But God is not finished with the devil and his henchmen just yet. After the 1,000 year reign of Jesus, all humankind will have been educated and perfected.... brought back to the same state that Adam had at his beginning....with no sin.....these will then be ready for satan to test them one last time before God grants everlasting life to the faithful ones who pass that final test.
Satan's confinement is for the duration of the 1,000 year reign of God's Kingdom, in the capable hands of his Christ and those who were chosen to rule with him. (Revelation 20:6) But after the 1,000 years are up, satan is let loose for a little while to tempt mankind again. Only those who pass that final test will be granted everlasting life....the kind that Adam and his wife lost due to their failure to obey their Creator.
Only then is the rebel satan along with his minions thrown into the "the lake of fire" which is described in Revelation as "the second death". All who fail the final test will join him in a place where whatever goes in, never comes out. But is this fire literal? What else does Revelation 20 go on to tell us?
And I saw the dead, the great and the small, standing before the throne, and scrolls were opened. But another scroll was opened; it is the scroll of life. The dead were judged out of those things written in the scrolls according to their deeds. 13 And the sea gave up the dead in it, and death and the Grave gave up the dead in them, and they were judged individually according to their deeds. 14 And death and hades [the Grave] were hurled into the lake of fire. This means the second death, the lake of fire. 15 Furthermore, whoever was not found written in the book of life was hurled into the lake of fire." (Revelation 20:12-15)
So this "lake of fire" cannot be literal because "death and hades" are thrown in there too. It is called "the second death" because no one comes back from this death....it means permanent removal from existence for anything that is thrown in there. It means "everlasting destruction".
"Gehenna" is another term for "the lake of fire". Everything about a person is "destroyed" in that place.
Salvation is open to all people....no one is excluded because of any external reason. God judges the heart which inspires action on our part. Sin is the end result of our thought processes. (James 1:13-15) When we are educated in God's ways, we are under obligation to obey his direction....after all, it is all he has ever asked of his human creation.
So just recapping....there are not three destinations.....
1. eternal damnation
2. eternal annihilation
3. universal salvation
1 & 2 are different ways of describing the same destination IMV. Only the ungodly, the incorrigibly wicked or disobedient will suffer this fate.
Universal salvation is not 'universal' at all, but is conditional....it always has been right from the beginning. Everlasting life was assured as long as the people were obedient.
God doesn't have theories.....he has truth and his word reveals exactly what happens to all of us and why.
There is everlasting life or everlasting death for mankind.....that's it.....not really more complicated than that.