Filed a SAR on customer who had three business accounts, moving money between accounts here and at other bank. The activity is minimal now and only conducted in one of the accounts at our bank. Should I still include the previous accounts in the SAR since it is a continuing one? Or, remove them from this report and they can look at previous SAR for more details.

I would include what you mentioned above in the narrative since this is a continuing SAR. So yes, I would mention them in the narrative but also saying that the activity is just being conducted on one account now.

And hopefully, this is your last SAR as all of the accounts have been closed.