Isolation devices- to use 3 or 4 footers?

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.