Switched outlet on when lights are off
A 14/3 with a 3-way switch is going to be:

Traveler 1
Traveler 2
Either hot (in) or switched hot (out)

That means you don't have a neutral, which is a must for a receptacle (switched or not).
However, there might be a solution if you can use smart switches. Many 3-way smart switches only need one traveler. You could, potentially, install smart switches and then rewire things so that you have:

Traveler
Hot
Neutral

and pass the hot &amp; neutral on to a receptacle. Doing this could get a bit confusing, so if you want to look into this, before you disconnect anything, upload pictures of the wires going into each of the 3-way switches.