Identify The Cell Address Of Last Change Made In Sheet Using VBA?

I am using an event macro to trigger a set of calculations. In the spreadsheet, users have a choice of 3 input cells to work out an answer; these 3 cells work out the same cost from different angles depending on the variables available to the user.

Users only ever need to fill in one of the three cells to work out the answer as the event macro I designed, should (upon input by the user), work out the remaining two input cells using logical arguments.

The macro and calculation work fine, except in the event of users amending a value in an input cell previously updated; in other words, if all 3 cells contain a value and 1 of them is being changed by a user.

The issue: I need the event macro to recognise the location of the last change made by the user in order to determine which 2 of the 3 cells need to recalculate. However, users will invariably press enter or tab etc... upon making changes and this has caused me issues so far.

What I have tried already:

1) running another event macro "on selection change" to record elsewhere in the model all cell selections as and when the user interacts with the model

EFFECT: it made the "on change" event macro very slow and clunky; so I need to avoid
2) declaring a range as set r = activecell.address

EFFECT: this did not work as the active cell's address would in fact be the address on the last cell; e.g. the one selected further to pressing enter (often the one directly below the cell recently changed)

A solution to add to the existing "on change" event macro that identifies the last edited cell