I'm with Asa, "in general" and "I don't know what you had in mind with the three foot high boulders...". You haven't given us enough info or your thoughts; I'll give you a couple of "ifs". If you are trapping for cats boulders usually are a good thing. If you are in an area if lots of boulders setting around them can pay off. If you have a set made close to a boulder and you get a fresh snow, e.g., one that drives the prey critters into and under the rocks, the coyotes will often work in close especially if the boulder is a bit over hung so as to leave a patch of bare ground at the base.