But all of them share 1 row which has a reference number for that same person. Could I use this to combine all of them into one csv file? The largest CSV file is 150k rows. I understand this might take weeks of credits etc., which is fine. I just want to make sure this is possible.

Here's a further explanation in case you need it.
File A might contain "reference number" in column a and column b is the address, city, state, zip, phone number.
File B might contain "reference number" in column C and ownership name in column a, cell phone number in column b etc. This is the largest file because this one has owner a in one row and owner b in another row and owner c in another row and each one will have the same "reference number" so on.
File C might contain "reference number" in column B and annual sales numbers in column a, monthly advertising budget in column C and so on.

My goal is to combine all of these 3 files into one large file. I understand that the same reference number would have multiple rows for the same reference number. That is totally fine and acceptable as long as all of the information is copied over for that reference number.

If you need further explanation please feel free to tell me and I'd be happy to email you about it.