Even if we isolate the last console cycle ... from 1985 to today (that's almost 30 years), three consoles have only been successful at the same time for maybe 4 years of that period?
Wii + PS3 + 360 were all selling fairly well from 2007-2010, that's about 4 year period out of 30 years. That's not exactly great odds for a three console setup.