Are any of the following correct?
It's all too-easy
It's all too easy
Explanation/citations would be greatly appreciated
There is no need to hyphenate the phrase unless it is used either as a compound noun or as a compound adjective.
As a compound noun, it is likely to be a nickname, in which case, capitalise the initial letters:
As a compound adjective, it would need a determiner before it, e.g.,
In the plural, eg 'all-too-easy solutions' no determiner is needed, as Kris commented, below.
There would be no hyphen involved. I assume you're thinking of the linking of two adjectives, but that's only when they come before a noun. If you want to read more rules about hyphens, read here: