Not the shortest formula...but, no Ctrl+Shift+Enter required:
=SUMPRODUCT(--($A$2:$A$51<>""),--(COUNTIF($A$2:$A$51,$A$2:$A$51)>=3)
,--(MATCH($A$2:$A$51&"",$A$2:$A$51&"",0)=ROW($A$2:$A$51)-ROW($A$2)+1))