3 days a week is plenty whether your goal is to build muscle or increase strength. This is all most beginner to intermediate lifters need as long as it is a proper routine, using proper lifts and utilizing principles such as progression, proper rest and adequate diet.
Just some food for thought, there are some people, Stuart McRobert being one, who suggest that a natural lifter workout out 3 times every 10 days as opposed to 7. While I know this is an extreme point of view, he is putting an emphasis on the fact that natural lifters need adequate rest and recovery times in order to progress.
Also, this doesn't mean 4 day routines don't, can't, or won't work, because that's not what I'm saying. However, when addressing the question "is 3 days a week enough?", I personally have to say yes, it is more than enough to achieve your strength and muscle goals.
"The fastest way to get strong muscles is also the fastest way to make them bigger. We know what works best to make them strong in the shortest time possible. If you want to get big, do the thing that makes them strong." - Marc Rippetoe
Zdravko Veselin Gaeta
I am not a powerlifter nor am I a bodybuilder. I just want to be big and strong.