It's one of Nintendo's biggest money-makers these days, so they'll definitely come back for a fourth one.
as i read in a magazine i bought at an airprot (im sorry i dont know which one it was, but it was a nintendo magazine), they said the next game is possibly going to be called super smash bros scrap, then again...i could be very wrong, thats all i know. sorry :D
the smash bros scrap thing is very unlikely but there is very very good odds on there being a 4th smash bros like he said nintendo makes so much money on it
they should make one also for the nintendo ds while they're at it. well i say if they do make one called that, it would have all the charecters that were scraped for previous games. like rayquaza and others