I don't know if stacking is how I would put it, but they are additive.
Dread Lords (DL) is the basic game. Good on it's own, but once you play the expansions you'll never look back. If you should want to, you can still play DL even if you have one or both expansions.
Dark Avatar (DA) adds quite a bit of new material to DL. Various inhospitable planet types requiring research to use, asteroid mining, Super Abilities for different races, and tons of other stuff. DL is required to play DA.
Twilight of the Arnor (TA) adds even more onto DL/DA. Each race gets a separate tech tree, building list, ship modules, etc. Both DL and DA are required to play TA.
All three titles contain a campaign mode and sandbox mode. When you start the game from the launcher, you have the option of "Switch Chapters", which will toggle you through DL/DA/TA (assuming you have the last two), so at any time you can go back and play any version of sandbox or campaign mode.