Why 3 diodes in parallel?
Because of the lower junction temperature compared to one single Schottky. Look at this link:

Nexperia Application note AN11358 - Parallel Schottkys as secondary rectifiers in flyback adapter: power losses and junction temperatures

From the conclusion in that application note:


  There are two main causes for a reduction of junction temperatures of two parallel rectifiers compared to a single one:
  
  a. Electrical power losses are reduced due the nonlinear VF versus IF characteristic of Schottky rectifiers
  
  b. Thermal resistance junction to solder point Rth(j-sp) is reduced because of the doubled amount of silicon, leadframes, leads and die bonds, resulting in a smaller total thermal resistance from junction to ambient Rth(j-a).
  
  On dual layer PCB with 0.5 cm2 mounting pads, thermal simulation results show between 73.5 °C and 79 °C junction temperature for two parallel PMEG4050ETP Schottky rectifiers, depending on peak current conditions. This is a temperature decrease of about 20 % compared to Tj  of a single rectifier.
  
  Both the reduction of forward power losses and the lowered Rth(j-sp) add to more or less 10 % decrease of junction temperatures in a 2.5 A total forward current application, independent from the PCB layout.