1. Make your first call using the Nxa-App then the screen shot below will appear
Select the add button +

2. The following screenshot will appear
So this will give you the option to choose either you add one of your extensions listed or you want to dial the number to add

Once done click on the options mentioned either "Add to the call" or "Add separate"
1. Add to the call
Means that the call that is being called its going to be added straight to the ongoing call
2. Add separate
It allows you to add the third call separate by talking to the person before adding to the call