I have some Newer cheap plastic ones that work fine so long as flare isnt an issue (they're uncoated). One had a defect in the dye, so its unusable, but the others are good. I also have some Cokin ones that are better made, but don't produce better images. My point being, Lee is a good brand and even if the super cheap brand works well (minus the QC issues), I wouldn't think they're be any problems with a Lee.
As for if you need 3 or 1, that's kind of up to you. If you buy the 3, you'll likely use all 3 at different times. If you just buy the one, get the one in the middle (I'm guessing 2 stops?) and it will probably work well enough for you in most situations provided you don't shoot a lot of sunrises/sunsets with slide film.