Why does Apple have 3 icloud mail domains?

It is the history:

See: iCloud: About your @icloud.com, @me.com, and @mac.com email addresses - Apple Support

My first Apple Mail account has been a mac account for many years. Then I started to use Mobile Me and had an additional @me account. And when Mobile Me had been dropped by Apple I had a new @icloud account.

LACAllen is pretty much right. Here is a linked support article: https://support.apple.com/en-us/HT201771

Essentially, @mac.com is legacy users and was in place from the debut of osX to late 2000s. It required a annual paid subscription to have an email address, lol. Then the short-lived mobileMe era happened, which lasted only a couple years before Apple retracted and replaced it with iCloud, a much more sweeping service. MobileMe was also a paid subscription and included primordial versions of photo sharing and web hosting, etc. The iCloud era starting in 2012 finally ushered in free email addresses and free operating system updates. That's when the business model of large tech companies turned more into user accumulation wars to see who can attract the most subscribers and retain them in their ecosystem of products.

The end result is that Mac fans who kept an original email address, now have all those versions linked and forwarded to their iCloud service. And it shows up in your settings as being able to receive from each of those domains. New subscribers will only have the one option of an @icloud.com address.

Not really an answerable question.

Historically, Apple has used @me.com and @mac.com as email domains. If that's your question, they are not icloud domains.

I believe those domains have been grandfathered to a degree, so you can't start to use either of them today, but will function.