You need to use the word and, not &. Also, while Harlowe will try to figure out what you mean (sometimes even correctly), it's best if each expression is complete when you join them.
(if: $kiss > $compliment and $kiss > $insult)[...
If you need to compare a whole lot of values and see if one is higher, you can use the (max:) macro as a bit of a shortcut:
(if: $kiss > (max: $compliment, $insult, $something, $anotherthing))
This will return whichever number is highest in the (max:) macro and compare just that number against $kiss.