ah, also just looked up halfling sligner, when they use that particular 3-attack ability, they have a -2 to attack. So if they also happen to have combat advantage, the penalty counters the CA bonus, so their attack bonus is still +4 to three attacks in this mix of situations of both abilities overlapping. So we're still talking fairly low accuracy.
Basically, you're talking about several conditions that need to be in place for this to work as "deadly"
a) their 3-attack power is recharged/available
b) they hit (+2 to attack, or +4 with CA)
c) they have combat advantage (to give the extra +1d6 damage) (a very low chance as the slinger needs it's own allies to use status effects, or the PCs to do something that makes themselves vulnerable)
d) They're not denied their action (with conditions from the PCs such as death, stunned, etc. ) - though there is a low chance of 1st level PCs doing any such deniying condition other than death ...
e) if a foe is next to them, they have to be willing to deal with the opportunity attack.
f) they have a clear shot to the PC (cover, concealment, terrain features, PC's allies, etc could all make the "to hit" roll much lower)
some of those things are likley while others have a low chance. it just seems like a lot of factors have to go in to allowing 3 hits with the extra damage.
but, again, it's been a while since 1st level so i could just be remembering the actual lethalty wrong.