Not to be a wiseguy, but if you don't know what they do why do you think you need one. And if used they wouldn't necessarily be used at the beginning of the chain.
First, I never said that I didn't know what they did. Second, you can place a compressor anywhere. You can place any pedal anywhere. Not to be a wiseguy, but if you're not going to answer the question then why even post a response?
Since they perform very different functions - what are you trying to correct? And yes they can be placed anywhere in the chain, but again it depends on what you are trying to correct.
I found this; Noise gate vs. Noise Reduction - Home Recording forums I was actually searching for one too tonight, I found that the ISP Decimator I would work best for my needs
Here's the thing though--if your noise problem is a high pitched hiss, that needs a different solution than a buzz or a hum. If your noise problem is coming from the bass, that needs a different solution than if the noise is coming from your other pedals. If you aren't sure exactly where the noise is coming from, you need to sort that out before buying anything. Let us know exactly what noise you are hearing, and where you think it is coming from, and we can suggest the most ideal solution. I can tell you right away that the answer is probably "have your bass properly shielded" and/or "use an isolated power supply for your pedals".
Alesis 3632 I have a noise gate on my compressor. It has come in useful as my Timmy clone (Dano v1 cool cat) is really noisy in conjunction with some pedals, and sometimes even by itself. I adjust the knobs on the gate until the noise is less or gone and the tone isn't affected. Sometimes a compromise, sometimes not. It's all very scientifical.
I use a Boss NS-2 but you have to hook it up correctly in the chain for it to work. I too found the light at the end of the tunnel in a You Tube video I only have 3 pedals on my board this a Boss CS-3 and a DI amp modeling pedal
I run a B7K on mostly full tilt in my current band, and while it doesn't happen often, every once and a while I can generate a nasty feedback loop...so to fix that and keep things quiet when I'm not playing I got a TC Electronic Sentry noise gate. I haven't played with it too much, but so far it works well.
Never used a noise gate, never needed to. I select all my pedals (including my compressor) to not make undue noise. @rtslinger i'm curious why you feel the need to use a supressor if you only have three pedals. Is your comp that noisey? Or your bass?
My CS-3 compressor is most likely one of the nosiest compressors on the planet but I like it and I have it for many years. many people mod them for this very reason. How noisy is it, It is so noisy I couldn't play any of my single coil pups using it. without the tone knob bury to the full bass side and little to no sustain. The NS-2 reduced it to a point where it was usable and that is not only great for that, but some club wiring and neon lights can interfere with your equipment this helps bury it as well.
I think in order to assist with an answer it would be helpful to understand the reasons why you need either one. And there are some good suggestions above as well if that is your situation. Many many years ago I used to use a Boss NS2 pedal as well to tame my Boss CS-3 compressor (which was extremely noisy) and Ibanez tube screamer (my Billy Sheehan days lol). The NS2 is a fantastic pedal and works really well.
My current signal chain is very silent. I use a noise gate (ISP Decimator) at extreme settings when I want a harsh tone with brutal cutoff, typically in metal situations. In the past I have used the NS-2. When used properly with its loop, it is a very efficient tool.
ISP Decimator for brutal, brick wall noise gating. NS2 for looping through a noisy amp (looking at you Peavey 6505+) and reduction in hiss TC Sentry for a combo of both i use the sentry.
Some thoughts on the topic from Analogman... The above taken (with a sodium-chloride granule) from Analogman's FAQ page.