Have multiple NAS devices, want them all synced. Best solution?

Sorry if this is the wrong subreddit to post this in. Let me know if there is a better subreddit for this question.

I have 3 NAS drives (Drobo's - I don't recommend them, but it is what it is) and they mostly have the same information on each one).

Bottom line, I want all of them to have identical info on each. (i.e. I upload a file to one of them, it will later sync to the others automagically or manually, I don't mind doing a monthly manual run of whatever).

What is the best way of doing this? Free solution would be better, but I know you get what you pay for.