No, intersex people are almost never exceptions to the rule. Almost every intersex condition is occuring only in males or females. In 99.98% cases we can easily say their sex, and only in 0.02% it is harder, but still possible with just a few tests. While only in around 0.001% cases it is very hard to tell, but in those cases in general it does not matter at all much, and they should be just called as they are used to live or looking like, instead of trying to use those 0.001% as "gotchas".

If you want to support intersex people: https://www.dsdfamilies.org/charity

5 methods that are used to determine sex of a person, works for everyone, including all intersex cases (except one, which was understudied, so it is unknown reality of it): https://twitter.com/AlexAlicit/status/1346223299571961856

And lets not forget that EVEN if they would be "3rd sex" (which are they not), then it is still not proves that "sex can be changed from one to another". It will make transwomen "3rd sex" or "4th sex", it will not make them women, it will not give them right to be grouped with women and enter women's spaces.

In societies with 3 genders, where gay men are put as "neither men nor women" and doing "women's work" - they are not grouped with women and have their own 3rd gender spaces, different from women's and men's spaces.