Hello everyone,

I'm working on a campaign set in the region of Calimshan in the Forgotten Realms. As many may know, Calimshan has a heavy "Arabian Nights" theme. In my campaign, I plan for the characters to find and (perhaps unintentionally) release an efreeti. The efreeti, Nezzal, has the power to grant any mortal up to three wishes at any time (unlike the Monster Manual's "3 wishes per century or more" rule) and is going to be the campaign's protagonist.

Nezzal is the kind of genie who will take the words of a wish and twist them in a way that is undesirable to the caster (example: wishing for invulnerability turns you into solid adamantine), and will try to trick mortals into wishing for something he can distort with word-play. Through the course of the campaign, I intend for Nezzal to appear to a character whenever they are alone and offer them a wish. He will also do this when in combat with them.

So here's my problem: everyone in the party has three wishes that they can make (resulting in at least 9 wishes total), and can wish for whatever they want (except for the classic rule that you can't wish for love, death, or more wishes). I'm worried that one or more of my players (who are very familiar with the concept of twisted wishes) may have their characters wish for something that may either take the campaign in a direction I don't want it to go (example: them treating the terrasque they accidentally summoned as a more major villain than the efreeti), or simply ruin it by wishing for it outright crazy and out of character (example: wishing for everyone in the world to sprout antlers). I don't want to simply say "No" when they wish for something like that because then it tells them I didn't prepare for that, and I'm worried that they might make a wish that is to difficultly worded for me to twist. Even then, what if the only clever way to twist one of their wishes would mean killing the characters (I have a group of only three so one or more of them dying kind of kills my campaign).

I ask for some help for avoiding utter chaos with a villain who can grant wishes at will. Note that he cannot grant a wish unless he allows it (he must grant permission before hearing the wish, so he can't decide to cancel after hearing it).

What I would recommend is to make your players make the wish at the table when they chose to use and make it be a simple sentence.

So, I think there a few things that you are going to have to accept if you want this efreet to exist for your party. 
1) You swear away all control of the campaign
Due to the fact you're giving them the ability to wish anything with very few limitations your players will deffinitely wish for anything and almost definitely something that you're not expecting. If you really want to do this you have to be ok with your players destroying whatever you've set in front of them or completely doing something weird like making everyone having antlers. 
2) You are never as clever as the players on the spot
Your players who are familiar with the whole twisting of wishes will take their time making their wishes "fool-proof". Due to this you most definitely shouldn't grant the wishes on the spot but have the efreet have a "delay" on the wish being granted to give you time to twist it. On top of all of this if your players want to do a wish make it so they HAVE to write it down that way you can have an accurate way to twist it and can refer to it when they complain about the twist.
3) TPK is plausible
You're right with your concern of the only way to twist a wish is to TPK your players. If you don't want this then just allow for that one time for the wish to go fine for the sake of the campaign.
4) The Efreet won't be the BBEG
Due to the fact the players have an opt in factor with the Efreet once they find out that it is malicious they'll almost never use it unless push really comes to shove. Due to that the Efreet will be more of a dark cloud hovering over your players than being the lightning and thunder that they battle against.