I would use the second variant "Can you understand what I have just written?" because of the word "just" which goes with the Present perfect tense, but I think that some Americans might also use the first one. I don't like the word "get" so much. It would be more appropriate in a phrase such as "Did you get that?" in informal conversation, such as on the phone when you want to check if someone heard you correctly.

All 3 make sense, perfect grammar :)

more common to say "Does that make sense?"