Hi. you could do something like:
Set Field [completed] =
_List = List (CEOcheck;CFOcheck;COOcheck)
If (PatternCount ( _List ; "Yes") = 3; "Yes"; "")
PatternCount () will count how many times the "YEs" is in the given text, in this case a list of the three fields.
Your If formula would be:
If [ CEOApproval = "Yes" and CFOApproval = "Yes" and COOApproval = "Yes" ]
put that in a script and your send mail step can go within that IF statement