Power supply testing using dummy loads
I will not have access to an electronic load and so thought of using a
  dummy load to mimic the load the supply will see. I'd like to clarify
  if I have the right idea of how this works?


This is the way most people would approach this.


  Am I correct in saying I will need to calculate the sum of current
  draw from the individual components and then use Ohms law to pick a
  dummy resistance to simulate this condition?


The dummy loads are chosen with this method and for your worked example the duumy load would indeed be 2.083 ohms but,, if you are designing the converter to supply 3 amps then test it at a 3 amp load is my advice. It's always a good idea to know you have the spare overhead capacity just in case you need it. I'd go for a 1.67 ohm load and this will produce a power of 15 watts and to prevent the resistor from exceeding its maximum limits I'd consider a power rating that is 1.5 times this value i.e. 22 watts.