clarification of bank holiday entitlements

A common problem is what to do if your 3 days are flexible i.e. vary from week to week. Doesn't seem to be much clarification on how to decide what to apply, but best I've come up with is that it depends on what you worked the week before.

Would be grateful if someone could clarify the following:

Have to sort out annual leave entitlements for 2 staff and that includes their bank holidays entitlement.

They both work 3 days per week - with one day overlap.

Person A - works Tues, Weds and Thurs so has only benefitted from having St Patrick's Day off.
Person B - works Mon, Weds and Fri - so todate has had all bank holidays off (except St Patrick's Day which was on a thursday)

Feel like I should give Person A a few extra days and deduct a few days from Person B.

Would anyone have a formula? Thanks.