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.

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.