What is hidden in 3D image?

Phil had just joined a club after his friend had recommended it (being a member for quite some time). They were sitting at the bar having their beers when someone yelled "21" and there was a small uproar of laughter. A few minutes later someone else yelled "34" and another roar of laughter rose up. Phil, confused about this asked his friend "Why is everyone laughing at the numbers being called out" His friend said, well we've been telling the same jokes for so many years that we just numbered them all and if you want to tell a joke you just call out a number" Phil nodded and said "Can I try?" His friend nodded and Phil called out "121" and everyone in the club roared with laughter and it didn't die down for at least another 15 minutes after. "Why did everyone laugh so hard at that joke?" Phil asked. His friend said with a small chuckle "We haven't heard that one before."