it sounds similar to how AIB calculate it, though they accrue daily and then add the interest back onto the account every quarter on the 16th. I assume you pay capital and interest payment monthly, and then every 3 months the interest is added to your account or you mean BOI only calculate the interest on the balance at the end of the 3 month period?
if its latter, it should just be the closing balance on this date* rate* number of days in that period. if its the former, have a line item for each month showing opening balance/interest/closing balance, then on the month you have the interest you need to split in 2, first part of the month on the old balance, second part on the new opening balance.