How do I use countif with multiple conditions

Column B has dates
Column C has Various data or Blank

I need to count the number of non blanks in column C if column A is one
of 3 dates

I have tried the array formula below, but am getting the wrong answer

{=SUM((Sheet1!$B$2:$B$100=A6)*(Sheet1!$B$2:$B$100= A7)*(Sheet1!$B$2:$B$100=A8)*(Sheet1!$C$2:$C$100<" "))}

A6, A7 and A8 are the cells witth the dates in


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Paul Sheppard


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