“Hit it off” is only used to describe what can happen the first time you meet someone — namely, you meet someone and you immediately find that you get along with them really well. Example: “I introduced Joe to Sam last night, and they immediately hit it off.” “Get along well,” in contrast, can be used even for people who have known each other a long time. “Click” is often used like “hit it off” to describe immediately getting along well, but it can also be used the other way. The image underlying “click” is a piece of machinery in which one part snaps (with a clicking sound) quickly and perfectly into a tight fit with another part. Applied to friendship, it means that two friends are perfectly suited to each other by personality.

Same meaning really: "We get along very well - I get along with her". "We really hit it off" - "We really click" Click is maybe a bit more colloquial....

Get well along" is very formal and correct in written speech. "Click" is a little bit less formal. Hit it off is even less formal or more casual. same meaning, different styles of speech. First one is equivalent to honorific in Japanese language. Click is polite. Hit it off is casual or informal but not impolite at all.

The correct way to ask is "What's the difference between..." And they all have the same meaning

All three could be used interchangeably.