lol. A difficult question and really a matter of personal preference. For me, I prefer going through any game series chronologically. In a lot of cases, it's fairly difficult to try to go back to something older in a series and you're possibly working with dumbed-down graphics, sound, control, etc., than the newer game you just played. I know I had that trouble when I played Oblivion first before Morrowind. While I now think that Morrowind is a better game, RPG-wise, story-wise, and exploration-wise, it was hard to get into because the graphics, especially the characters, were nowhere near as good.
It really depends what you're going for. Admittedly, the Descent series is not much more than action games with little backstory. The gameplay feels different between Descent 1/2 and Descent 3, but it's similar enough that you probably won't be missing much. Descent 3's environment is definitely more varied than the previous ones. But you're talking to someone who went ahead and played Doom 1 after playing Doom 2, even though they're essentially the same.
Since there isn't much variation in the gameplay in the series, my advice is to put it down to how much you like it. If you can't get enough of Descent's gameplay style, get the whole series and start with Descent 1. You'll have lots of different missions to satisfy you. But if it looks like you may tire of it after a while, just get Descent 3 then.