Both the methods are correct.
Let us assume the following:
Scan time (active power mode) for 1 button = ts1
Process time (active power mode) for 1 button = tp1
Current in active power mode = Isp
Then, for 3 buttons, you could either split it as scan button1, process button1 etc.,. or as scan the three buttons and then, process the three buttons. (i.e) ts1, tp1, ts2, tp2, ts3, tp3 or ts1, ts2, ts3, tp1, tp2, tp3.
It depends on your firmware implementation.
Thank you for your answer.