Currently, I'm exercising 3 days a week (every other day), each day doing 2-3 muscle groups. Currently, my goal is to increase muscle mass. My question is, given my exercise goal, is it better to really stress a given muscle group for a single day and then "ignore" it until next week, or to work out all groups each day, but to a lesser extent?
The answer to your question is yes. Yes, you can do a 2/3/4/5 day split each week. And, yes, you can do a full body workout 2-3 times a week. In fact, I would suggest doing both - just not at the same time.
Most people who go to a gym will do a split routine. Usually a 3 day split involves:
You can also add in core muscles on any or all of those days. Alternately, you can do a full body work out. A workout of this type will involve exercises that engage most of the muscles in your body:
You cannot do both these routines at the same time, it would be way too much. You can alternate them however. Do a 3 days split for a couple months, then switch to a full body routine for a while, then back again. This will challenge your body in different ways and will keep you from getting use to your exercises and plateauing.
Your muscles increase in size during their recovery, not during the exercise itself.
Therefore, you should not be working muscle groups on consecutive days.
I generally find 2-3 times a week is ideal, on non-consecutive days.