Firstly, the 3x5 is a program rather than a routine. This basically means that it shouldn't be judged by a single snapshot or a single workout and is meant to last over some considerable number of workouts. While the exercises are chosen for maximal impact, its benefits arise from the ongoing progression rather than the mix of exercises.
During the early weeks, if you feel underworked, do a little bit extra. Don't, however, blast yourself to exhaustion since that'll eat into recovery. This pretty much applies to anyone at any stage in their lifting. You will get some days when you feel on top of the world and have energy to burn. Feel free to burn some of it.
I recall one workout where I was just catching my breath after my last set of deads. I remember thinking that I still felt OK and was considering doing a few sets of curls. Reason took hold at that point and I did another set of deads then went home.