Isolation devices- to use 3 or 4 footers?

The number doesn't matter much other than 4 (asuming a rectangular footprint) will be more stable.  3 can be used to deal with uneven weight distribution.

More important is that each spring is compressed with the right amount of weight.  For example, the cheap spring bases you can buy on amazon have 7 springs in them but you can take out springs and use 3, 6, or 7 springs to give you a stiffer or lighter effective spring.  You want the spring to be compresssed about to the center of it's travel ideally, but I'd say 25% to 75% is fine. 

More expensive and well engineered isolation spring devices will have a design load specified by the manufacturer so if you gear weighs 40 lbs, and you use 4, you'll need 10 lb rated devices.

So a time to use 3 would be if you have a component with more weight in the back like a tube amp with transformers mounted on the rear.  for a 45 lb tube amp you might buy 15 lb isolation devices and put 2 in the back and 1 in the front.

Jerry

Four, primarily for stability. When changing out cables or fiddling around with your system, having four footers reduces the chances of a component tipping.

You can position them a little asymmetrical, as needed, to account for unbalanced loading (i.e., a big transformer) and, as discussed by @carlsbad2, whether springs, elastomers, etc., account for the proper weight per footer.

If rigid isolators, 3 would be recommended.  With 4 you may have a soft foot(non contact).  
 

If the isolation foot gives, like spring configurations, then 4 can provide more stable support.  
 

Try to align the weight of the piece of equipment to the total capacity of the isolation footers.  

My DAC only has three points coming out of the bottom for it to stand on, so I have three Isoacoustics Orea Indigo’s under it. My preamp has four feet and is bigger, so I went with four isolators under it.

Anyone here use the brass cones from Live Vibe Audio? OCD Mikey raves about them. He's tried Townshend springs and finds the brass better. He hasn't tried Stacks yet. 

Innuos uses 3 footers. Maybe there is something to it.

I prefer having four feet on my gear that isn't rack mounted. But consider that it's the three-legged stool that never wobbles. It's why photographers use tripods.

More effective, or sounds better?
I find 3 sounds better on my Garrard 401 tables. 

I use 4 footers on my Preamp and 3 footers on my Amp because my manufacturer used 4 footers on my Preamp and 3 footers on my Amp.

That being said, I slightly off-set one of the footers to help mitigate any tiny standing-waves that the electronics transfer to the Aluminum casing (which, in-turn could be reflected back into the electronics). 

I use inexpensive footers, and they seem to work very well (converting motion into heat) - the improvements are obvious, not subtle.  My system is posted.  

My only advice may be to start with something inexpensive and see if you are satisfied before going high-end.  If the inexpensive footers are not satisfactory, expensive isolation footers will work very well (from all reports).

Very few components are physically balanced. I place one directly under the transformer (typically the most vibration to drain) then 2 or 3 more depending on weight/size of component

FYI A couple weeks ago I replaced el-cheapo iso devices from Ali-E with Very Black Hole devices from VeraFi Audio LLC to fabulous affect. Now have the huge life like 3D sound stage Great price $199/4. They have lots of cool stuff at great prices

Just starting to use footers myself. Started with my TT, that has 3. 

From what I have been reading (lots of white papers, feel like I'm back in school) the number of footers doesn't matter much, or really what you use. What matters most is that what you use is in the weight spec of the device.  So if the device is 10lbs and you are using 4 footers you want to be in the middle of the weight spec. so you would use footers that support 1-3 lbs for that 10 lbs device. 

Think the main reason for using more or less footers is trying to find that middle weight spec, and/or cost. Buying 3 footers will save a lot of money if doing several pieces of equipment. But also, anything that is a little heavy, should have a solid base. 

Anyone here use the brass cones from Live Vibe Audio?

Yes, I read their stuff years ago and tried them under my speakers and components but did not discern any sonic difference between those and other spikes/cones such as edenSound or even the bog-standard Ramset spikes that come with Sound Anchors stands, which were in fact way easier to adjust. However, many here like the Live Vibe Audio stuff so the magic may be in their steel platforms that interface between their cones and your components or speakers. I have since happily moved to compliant footers, either springs or elastomers, and given up on hard cones or spikes.