Creating a vba macro that opens a msgbox if more than one cell is not empty in a range

I am somewhat of a beginner to macros and need help with solving this macro. I have 3 columns (col. G, col. H, and col. I) that a user can enter an "x" into any cells in each row for these 3 columns. However, I do not want the user to be able to choose more than one option per row within these 3 columns.

These are the 3 different options that a user can choose from and type an "X" in the appropriate cell under one of these columns in each row.
Column G = "Regulatory Obligation", Column H = "Hazardous Substance", Column I = "Fire Protection" (these are the titles of each of the 3 columns)
Rows = 8:100
ranges = G8:G100, I8:I100, H8:H100
Full range = G8:H100

I would like an error message box (msgbox) to appear if the user chooses more than 1 options (places an "X" in a 2nd or 3rd box). I would like this formula to work for all the rows G8:H8 down to G100:H100.

Msgbox = "ERROR: you have chosen more than 1 option. Please review the options and choose ONLY one." (or something like that)

AM_BPTemplate_USA-CANADA_v1.xlsm

Can someone please help me? I greatly appreciate it.

The reason that I am asking for help on solving this macro is because I am already using Data Validation with those columns. I have created a drop-down list for G8:I100 allowing the user to choose option 1, 2 or 3 for each cell that in that range. As far as I know, it is not possible to have more than one Data Validation for a cell. If there is, I am not aware on how to do it. This is why I figured a macro would be used. What do you think?

Can someone help me with this? Please.