Are you comparing Today's date with the start date?
I suggest you use conditional formatting. Here is a simple sample. Select the cells that you need to apply conditional formatting > click Home tab > Conditional Formatting > New Rule > Use a formula to determine which cells to format.
Enter following formulas and set the cell format you need.
Red: =IF(TODAY()-$B2>=91,1,0)
Orange: =IF(AND(TODAY()-$B2>=31,TODAY()-$B2<=90),1,0)
Amber: =IF(AND(TODAY()-$B2>=1,TODAY()-$B2<=30),1,0)
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