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.