Adding a light to existing 3-way
Not enough wires. 

You would need 12/2 from source to first light, 12/3 from first light to first switch, 12/4 from first switch to second light, and 12/3 from second light to second switch. 

But this is not the best practical way to add a second light to the original circuit. 

Originally, if I'm not mistaken, you had 12/2 from the source to the light, 12/2 from the light to the first switch, and 12/3 between the switches. 



Starting from that circuit, the best way to add a second light is to run a new 12/2 from the original light to the new light. This new cable need not, and probably should not, pass through the first switch j-box. 

The wires in the new 12/2 must be used to connect the lamps in parallel. In the j-box of the original light, you will find two wires connected together -- leave these alone. You will also find two wires connected to the lamp. Of these, the one from the source cable is the neutral -- connect the white wire of the new 12/2 to this. The other is the switched hot -- connect the black wire of the new 12/2 to this. 

At the new light, connect the white wire to the shell side of the light, and the black wire to the tip side. (The shell is the threaded part of the socket, and the tip is the little button in the center.)



Any other combination of cables is not going to be any cheaper than this, and will certainly be more complicated.