They're different classes of drugs, so they work on different problems. Lamictal is a mood stabilizer and I've been on it for years. It is a good drug which many people have luck with both alone and in conjunction with other drugs. The Paxil and Lexapro are anti-depressants and work to help elevate mood, so if you're depressed, the Lamictal won't really do much about that, but the other drugs will, hence the combo.
You also see mood stabilizers like Lamictal combined with anti-psychotics for controlling mania like Abilify or Seroquel.
Hope this helps,
Co-Moderator, Bipolar Forum