Can I make a stock with a mixture of beef, lamb and ham bones?

I agree with ChefJune. The ham bone is the one that stumps me. Although, if you use the stock for beans and hammy soups it will be great. If you separate the ham bone, you'll get more mileage with two stocks.

I would use the beef and lamb together, but not the ham. I'd use that separately for a bean soup. I think using all three you'd end up with a mishmoshy flavor.

I do not see anything that prevents you from making a soup from diversified bones, except perhaps for the perception that beef + lamb + ham flavor combination is not exactly the most straightforward one. But hey, cooking is about imagination and creativity and having fun, and being frugal, and discovering new flavors, so absolutely go for it!

I typically make a mixed meat stock and it's not a problem. In fact, this weekend I roasted chicken trimmings and the bones from a leg of lamb to make a dark pho stock. Yum!