Yes I do agree with that. Never pursue someone who is unavailable.
And just because they claim to be unhappy or going through troubles with their lover does not mean that they are available. I don't know how many times I've known about a guy claiming that he and his girlfriends were going through problems and might break up soon...that does not mean that he's single!
About the threesome, you need to discuss it first. My relationship is far too uniquely complicated to compare to...as was that situation. You need to decide ahead of time who is allowed to touch who and what kind of contact will be kept afterwards. Let me warn you though...if you make him think he can't touch her and then leave her and him in the bed together later...you won't really be able to come down on him for sleeping with her without you...because it probably will happen.