Would you happen to be:
Though I am only three letters, I can mean a lot.
Both children and adults all enjoy fun, and interestingly fun has 3 definitions.
I can be "educational" if I want or I can just mean a series of nouns and adjectives.
Fun can be used to increase retention of material in a learning environment; plus, fun is a noun, adjective, and a verb.
Everyone needs a little of me.
Everyone does need a little fun in their lives. This can take on different meanings to different people. For example, fun to me is working on my car or reading a book; fun to you could be shooting guns and riding roller coasters.
I can give you a glimpse of different worlds.
Everyone has fun using their imaginations at some point in life, this can allow you to not just see, but visit other worlds.
Or I can simply be something that makes you feel happy.
Fun is typically associated with happiness.
Another guess (this is a long shot):
H2O; though technically a number and two letters, it definitely meets all of the criteria.
Would you happen to be see?