First of all, I end up recommending this/these game/games but my overall score regarding this trilogy is barely 6/10 and I'd like to share my view about this.
Alien Breed: Impact - The graphic looks neat despite the age, the controls are a little clunky and the explosions shockwave, more than being realistic, just give you nausea after some time. I swear, the camera shakes all the time non-stop. Still, the levels are smart-structured and the weapons/weapons effects are very cool. The story begins to unfold here and it's quite interesting, so when the game ends, you fnd yourself wanting more because you really want to know what's next.
Alien Breed: Assault - By far the worst of the three chapters imho, the levels are just huge for nothing, you always end up running in small hallways or from a room to another, in quick sequence. The secrets/logs aren't really that secret so the exploration part of the game is also limited compared to the previous chapter.
Talking about difficulty and all, there's no reason not to play on Elite from the start, all you need to do is always have an automatic door between you and the enemies, since they will just stand there for a few seconds getting hit to death... or you can simply run past them. This is a huge problem shared in all the Alien Breed games, but in this chapter it's more of an issue because despite the levels being huge, you always end up running through small places (making your life easier or miserable depending of the situation).
Story-wise you won't be that much satisfied but I guess they wanted to leave the better stuff for the final chapter.
Alien Breed: Descend - Imho the best of three, even with all the flaws of the previous chapters, it has some increased pace that in gauntlet-games like these is a must-have.
The weapons at your disposal are better than the second chapter and more similar to the first. For the first time here, even tho the logs are still very easy to find, the secrets are not, you may have an easy time with one or two, but you're gonna experience hell to find a couple, and not always you can spend your time looking for them (there are a lot of events with a timer on, so no easy backtracking). The story kicks in to his final stages and before you realize you're on the last mission the games ends with a brutal end-game fight.
Now, talking about stuff shared between the three games, I have to say I really hated the camera and overall the stiff controls, they could've done a better job, sometimes you're forced to proceed with a fixed camera and it's really annoying.
The story isn't that original but it's cool enough, it only lacks a couple of secondary characters, since you're always on your own or talking to MIA (the AI android) more or less.
The biggest blackdot in these games is the lack of Local Co-Op, it'd have been that much better with this option... also, if you're an achievement-hunter, you're gonna experience hell to do the last achievements in multiplayer, problem being there are not enough players online (or none at all) still playing this trilogy, so you will probably need to find someone on the forums and ask for help.
As I said previously, overall my vote is 6/10, I recommend you to buy the trilogy on a super special discount or in a bundle (better) like a I did, since imho they're not worthy the full price (or even half the price tbh).