In this thesis we study saturated fusion systems on p-groups having sectional rank 3, for p odd. We obtain a complete classification of simple fusion systems on p-groups having sectional rank 3 for p ≥ 5, exhibiting a new simple exotic fusion system on a 7-group of order 7∧5. We introduce the notion of pearls, defined as essential subgroups isomorphic to the groups Cp X p and p+1+2 (for odd), and we illustrate some properties of fusion systems involving pearls. As for p = 3, we determine the isomorphism type of a certain section of the 3-groups considered