My review here will be on Homeworld: Cataclysm more than the other two, but much of what I have will apply. The Homeworld series of games, and Cataclysm specifically, I picked up because they were different. Unlike the 2D+ you got from some other RTS type games of the era, this had true 3D ship to ship combat as you had to specify both direction of travel but also elevation ‘up’ or ‘down’. If you had been playing hours upon hours of Starcraft or Command and Conquer, it was a welcome change.
The story followed a race (the Kushan) who found out they had been displaced from their homeworld long ago, and build a generational mothership to go take it back. The subsequent stand alone expansion and sequel deal with what happens after doing that. The other unique feature is that your fleet (based out of your mothership) is persistent from mission to mission. So there is a benefit to planning and executing a strategy in your missions. You may find your self hard pressed to finish the next mission if you get most of your ships blown up in the previous one. It might take a bit of time to gather the resources to rebuild.
The games (despite being 10+ years old) run well enough even on Windows 7 to play now. If you can find a copy and think what I said above sounds intriguing, check it out.