Show input box to edit a locked cell

I'm trying to build a fairly complex workbook containing many inter-dependent sheets.
For reasons of data, formula and sheet integrity I want the main user input sheet to be mostly locked, including some cells users may want to change. For example very specifically: locked cell A3 on Sheet1 may appear to be a simple number when in fact it is referencing (i.e. picking up) the value from another hidden sheet, say cell C3 on Sheet2.

What I want is if the user tries to edit cell A3 on Sheet1, instead of getting the usual Sheet Locked warning he gets an input box where he can put in the value he wants, which is then entered into the appropriate cell on hidden Sheet2 and then in turn picked up again and shows in cell A3 on Sheet1.

Is something like this possible?

I'm no programmer so if it is clear, idiot-proof instructions would be much appreciated. Many thanks.