Sometimes, with some breeds, strains and conditions that is normal. Were these chicks shipped? Also, if the vigor just isn't there in the bloodlines that can be as well. 3-5 out of 18 is a pretty high number, but there are a lot of things at play here, so don't start blaming yourself just yet.
A few questions...
How hot are they & is there any possibility of a draft in their brooder area? Are they pooping, do they show any signs of pasty butt, etc etc