Running power to a single switch from a three way switch
Take a look at how three way switches are wired.  (The actual physical layout of the devices may be much different, but this is how the connections work.)  



Note that the common from one switch goes to the breaker, and the common from the other goes to the light.  

IF the cable to your outside light is in the same box as the switch fed from the breaker

AND the circuit's neutral is in the box, as well as the common hot / travellers 

THEN you can tap the circuit ahead of the switches to power the light.  

It looks like this is probably the case.  It looks like the neutral is coming in on the same cable as the hot.  

Not sure whether this switch is the one fed from the source or the one that connects to the light.  You could verify by seeing if power cycles on the common terminal when you move the switch, but unless you're super confident working with a live box open, I think it's best to just try it and see.  No sense working around energized conductors if you don't have to.  

What you'll want to do is add the white to the white from the outside light to the splice with the other neutrals.  Also add the ground to the other grounds.  

I recommend the push in type connectors unless you've had some practice 
splicing with wire nuts, I think wire nuts are better once you have some proficiency, but the push in connectors are hard to mess up.  



You'll want to connect the common, the source hot, and the outside light hot with a pigtail - for this purpose, about 9" of #12 black wire:  


Remove the black conductor from the common screw.  
connect one end of the pigtail to the common screw terminal
splice the other end of the pigtail, the black that was on the common, and the black on the cable to your outside light, with a push in connector. 


(This picture shows a pigtail with a receptacle but it works the same with a switch terminal.)  



If you lucked out and the wires are in the switch nearer the source, your outside light will now be energized at all times.  If it switches on and off with the three way switches, it's in the wrong box and you have a bigger project getting it to work the way you want.  

The last step will be to add a single pole switch to the outside light.  

To do this, 


remove the black wire from the outside light from the push connector by pulling it lightly while twisting it back and forth.  
connect a new black pigtail to the other screw terminal on the switch.  
push the other end of the new pigtail into the connector with the other black wires.  


Sketch: