COUNTIF problem

Hello,

I cant quite figure this out...

I have a table that has Column A "Date of birth", Then for each milestone at Column B, C, D

I want to count the number of people based on year of birth that have achieved all 3 milestones in B, C, and D only (column B, C and D all have dates if completed, and I don't want to count the blanks or N/A's)

I've used =COUNTIFS(Sheet1!A:A,">=1/1/2014",Sheet1!A:A,"<=31/12/2014") to get only those born in 2014 but how do I go about the rest?

Thanks for your help!