I’m not a good english speaker, but i’m French, so i can tell you my opinion 😉

All your suggests are right, but the first one is more straightforward.
Except as i would prefer:

“Je vais prendre l’air. À moins que ça soit interdit, ça aussi ?”

The second is a little bit complicated and not very used in everyday language.

On the contrary, the third is not very beautiful.
I dont know how to say that in english, be in french it’s “familier, pauvre”.

1 , 2 and 3 are correct.

ce is less frequent.

more frequent is: à moins que cela (ne) soit interdit.

you can use them interchangeably.

2 is more stylish.

3: sauf si is more frequent in everyday speaking.

According to the existing answers and commentaries, the answer to that question highly depends on who answers. Given that I don’t agree with all I’ve read, I publish my own point of view.

My answer is that all 3 forms are correct when speaking. If you write, I recommand you to use the second form, which is the most correct and formal in my opinion. Personally, I always use “ne” when speaking even if it’s optional.

The use of “ce” is also perfectly correct ! It replaces “cela” (but the use of “cela” would be correct as well and a bit more formal). I won’t suggest you to replace it by “ça”, which makes the sentence more “popular” and is useless here.

To summarize:

• Most formal: à moins que cela ne soit interdit.
• Less Formal: à moins que ce ne soit interdit.
• Slang: à moins que ça soit interdit.

All 1, 2 and 3 are all correct.

Personal opinion: In spoken language, I would use 1 or 2, indifferently (2 sounds better, but 1 is shorter). In written language, I think I will systematically use 2.

Concerning 2: after “à moins que“, the “ne” (ne explétif) is usual. It is because “à moins que” indicates that the fact in the main sentence (“Je vais prendre l’air“) will happen except if the condition in the subordinate (ce soit interdit) is satisfied. The “ne” just highlights this negative hypothesis.

Examples:

• J’irai à votre rencontre, à moins qu’il ne pleuve.
• Ce travail sera terminé, à moins que quelque imprévu ne survienne.

Source: Université de montréal