Excel Formula to find conditional duplicate instances in a column

Hello Excel Experts,

I am trying to find the total number of duplicate instances based on user Input.
In the attached sample data, i have data in column A,B, C spread across 40K rows (even more). So based on category (1 input) and sub category (2 inputs), i want to find the duplicate instances of 'Item Number'. In the attached example, category is 2000, and sub category is A and D, so there is one 'Item Number' duplicated which is "AB1236", hence count will be 1.

Can anyone please help me with the excel formulas to pull the total duplicate counts.? All are Random values in column A,B,C and nothing is standardized.

Thanks for the help.