Not sure if it's what you want... but you can easily use an IF() formula to create a message in a cell based on conditions you set.
e.g. =IF(A1="Option 3","This is my pop-up message","")
this will only show the message in the cell you put it in if Option 3 is selected from drop down in A1.
You can Conditionally format the cell based on the condition too.. if you want to add colour, bolding, etc...
Easy and effective.
You can even use VLOOKUP to help you pick a different message from a table based on each option picked...
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