I have no idea why Mew is winning this poll. First of in a battle pokemon can only know 4 moves, there is only so much Mew can know. Mew has low stats compared to the other two.
Second of, I have no clue why people would vote for Celebi because he is technicaly most prone to the others attacks. Celebi is not only Psychic, but he's also a GRASS type which is WEAK against PSYCHIC type attacks. Celebi does'nt get damage reduction from psychic type attacks in that situation like the others do.
Jirachi is the clear and only winner in this matchup. Not only do psychic attacks 25% of the normal psychic damage, but hes also a STEEL type pokemon which also reduces damage to 25% of the original, unaffected amount of Physical damage that is weak against Steel.
Now if were talking out of battling and in a movie scenario. This is what would happen. Mew tries his hardest but looses badly. Mew is just a pokemon that knows every move. Celebi is a TIME TRAVELER! Even better Jirachi makes WISHES come TRUE! Mew would horribly loose. Then Jirachi would whoop Celebi because it can take a few moments and wish Celebi's time trave abilities away.
In the end Mewtwo would show up and demolish everything, and everyone. Mewtwo can actualy turn off Pokemon powers and abilities within a second. Jirachi would become as useless as a Metapod. Yes Mewtwo does have control of other pokemon's powers, and abilities.
However in this scenario in battle and in movie I vote Jirachi. That is if Mewtwo is not present at the time. See ya bye.