How to add a light to an existing 3-way circuit
When you have two switches in this configuration, you have to look at one important detail.  Power is supplied to one switch, and the switched power comes off of the other switch and goes to the lights.  In your diagram, "from source" wire leads to the switch on the left.  The switch on the right has a black wire coming from it that supplies power to the lights.

Because of this, the only way to add another light is to get power from one of the existing lights.  You can't pull power from the switch on the left because it doesn't have a wire for switched power.  You can't get power from the switch on the right because it doesn't have a neutral.  The only two boxes that have switched power and neutral are the two light fixture boxes.