Status formula based on mult. conditions

How do I write a formula that would change a cell with drop down 3 tier symbols (Red, yellow, green) for the following. This would be in sheet summary - if all priority items are on time, its green. if only one is behind, its yellow, and if two or more are behind ,the project is red status

Green: IF [Priority] is checked, and {Due date] is in the past, and done is checked

Yellow: ONLY ONE - IF [Priority] is checked, and {Due date] is in the past, and done is not checked

Red: TWO OR MORE - IF [Priority] is checked, and {Due date] is in the past, and done is not checked

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