VB6 convert MSHFlexgrid1 cells

Hi,

I have this MSHFlexgrid1 where in Column 0,1 and 11, I may have multiple rows with the same values. Everything is already sorted by those 3 columns.

Using this:

    With MSHFlexGrid1
     .ColSel = 11
        .Sort = flexSortStringAscending
        .ColSel = 1
        .Sort = flexSortStringAscending
        .ColSel = 0
        .Sort = flexSortStringAscending
    End With

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What i would like to do is that if i have multiple rows with the same value in column 0, 1 and 11, it will leave all cell populated  as it is and also add the letter "H" in column 4 of the first row only.
Then, it will empty the value in column 3 and add the letter "D" in column 4.

Example of the grid before the macro:

Then after the macro:

What would be the best way to do that as i can have up to 7000 rows in the grid.

I have put a sample of the project in attachment.

Thanks for your help