When I do readings, I would start by pulling one to three cards to see what's going on. Im complicating myself but it helps in terms of accuracy. For me, the first card that comes up is the main card as -- I understand that some people take the middle one as representing the main one. For me, it's the first card and whats next influences this one.
Examples:
3 of Cups + III the Empress = Celebrating abundance and fertility.
2 of Cups + XVIII Moon = The enchantment of romance...or A honeymoon. Sometimes the cards are very literal.
Something else I look at are the directions. For instance, a court is facing left can mean that a person is interested in your subject/querent...right will mean not interested or currently not a part of his/her life. If two courts are facing each other, it can mean showing interest in each other, being friends, or agreement with each other. If one court is facing and the other is looking away in a pair, it can represent one sided relationships.
Basically, I just go by what I believe the cards mean and with my intuition to try to see what the Universe is trying to tell me with those three cards -- I strongly believe the cards are trying to tell you a story. Sometimes, one cards can answer a question, sometimes two, sometimes three. Sometimes three won't do the trick either (once again, I go with my intuition -- I'd immediately get that feeling that there is more that the Divine wants to tell me, the reading would just feel incomplete to me like a piece of a puzzle is missing), so if I would pull more cards and i correlate them from the first one with the last one. But I always start with no more than three cards.
I wouldn't recommend anybody who is just starting to immediately start with more than three cards. Follow your intuition, what the cards mean to you and practice, practice, practice -- trial and error. There is no right or wrong way in the Tarot, it's all about what works for you and what doesn't.