So, you have two ways of assessing his cognitive language recognition.
First, as you have done-- having your son verbalize answers is fine. What you described sounds very similar to most typical kids I have worked with at this age. (I led 20-30 month group for a long time). You may see that some peer girls might have more expressive language than your son-- all kids progress differently and let me say with certainty-- if they aren't developing in one area, they are certainly developing in another. Some kids aren't very verbal but show interest in other areas of toddler development, such as sorting/categorizing items, building simple structures with blocks, and exploring sensory activities. Some kids express themselves very well socially in a non-verbal manner.
From what you describe, your son seems pretty on target to me. Kids, even up to six, won't always answer every question directly or successfully or even with a 'realistic' answer (esp. as they get older and their ability to create fantasies or stories develops... some parents mistake this for lying, which is utterly different, including a different motivation.)
It might be that you should become more familiar with your child's developmental abilities for this relative age. I should note that with developmental milestones, this is a 'general' window of time where these skills are developing. Some kids will present these skills anywhere between three months ahead or three months behind or more. Here's a good link:
http://www.allthedaze.com/development3.html
I also suggest for families who worry about language comprehension/acquisition-- do limit tv and computer games; focus more on real life experiences, narrating and talking together as much as possible, and offer language with rich books and songs, etc..
Some kids are later with their emergent language and so, if you still feel at three that there is something amiss, do address it with your pediatrician. At that point, they can give you a referral to have an evaluation done and if the therapist deems it advisable, your son could receive therapy. There are early intervention programs in most areas, this would be another resource to tap into.
ETA: re-read Isn'tThisFun's post. "m not even sure that asking "What do you want for a snack?" is an appropriate question to expect an appropriate response to. Most children, at that age, still need to be given specific choices. It is just too big a question. " She is REALLY right. I often suggest that we limit any choices of food to the plate and to keep choices/questions as concrete (meaning, there is a tangible, visible option present) as possible-- little ones often don't get 'abstracts' yet.