number of successes from one roll

I roll 3d6 and my target number is 15.  For every 1 that I roll UNDER my target number I get one additional damage causing hit from my weapon. So, a 15 is a hit, and a 14 are two hits, 13 three hits, 12 four hits and so on.  Is there a way to add this information to my roll macro so that if I choose to shoot several shots it can calculate how many damage causing hits there were? This is my roll template: &{template:GURPS} {{roll= [[3d6]] }} @{Juan Pablo Alejandro del Castillo de la Cruz|vs} {{target= [[16 + @{Juan Pablo Alejandro del Castillo de la Cruz|modifier}]] }} {{charactername=@{Juan Pablo Alejandro del Castillo de la Cruz|character_name} }} {{rolltype=ATTACK}} {{attackname=TK: Bullet}} {{damagetype=Piercing (pi) }} {{woundmod=1 }} {{damageroll=[[ ?{What level?|1}d6+?{pips?|1} ]] }} {{hit-loc=[[ @{Juan Pablo Alejandro del Castillo de la Cruz|calledshot_check} ]] }} {{critbody=[[ @{Juan Pablo Alejandro del Castillo de la Cruz|critbodytable} ]] }} {{crithead=[[ @{Juan Pablo Alejandro del Castillo de la Cruz|critheadtable} ]] }} {{success=([[16 + (@{Juan Pablo Alejandro del Castillo de la Cruz|modifier} +1))]]}}

You could try replacing {{roll= [[3d6]] }} with {{roll= [[{16-3d6, {0} }kh1]] hits }}