It just hit me what the easiest solution probably is.  It won't be  
quite as fast but...

Tell application "FileMaker Pro"
	tell database "myDB"
		tell layout "myLayout"
			set {fa, fb, fd} to get {field "a",Field"b",field"d"}
		end
	end
end
set lun to open for access someFile
repeat with i from 1 to count fa
	write ""&item i of fa&","&item i of fb&","&item i of fd&return to lun
end repeat
close access lun