we were wondering if we would have the compartment to our selves, or will they sell the extra spot to a stranger? thank you in advance for your help.
Yes, you are likely share the couchette with a stranger.
There are two ways to have a compartment to yourselves: 1. Book all three beds in a 3-bed sleeper. 2. Book all four bunks in a 4-person couchette and leave the 4th bunk empty.
If you book far enough in advance to get discount fares, #2 can be cheaper than #1.