Yes, you could. This is how breeders have been able to attain all of the sub-species of fancy goldfish that we have. But if you don't know what you're doing and aren't doing it with a clear knowledge of what you want to achieve by the cross-breeding, all you'll make is a mess. I would think very clearly about what you want to achieve by doing this, and as you don't even know what type of goldy the third one is, I don't think you're ready to cross-breed yet. I would read everything I could about cross-breeding fancies before ever attempting it to be sure that the result would be something new, unique and beautiful and not a smudged up mess. You don't want to end up with a "heinz 57 mutt." And then you have to remember that it takes many generations of cross-breeding to achieve the desired effect. Personally, I would attempt crossing a black moor with the blackest butterfly veiltail you can find to get a black moor with a long flowing fin. This particular type of moor doesn't exist anymore and it would be a beautiful fish. And then, of course, you need to know whether you have a male and a female and if they're of breeding age yet and even if you have all of these things in place, quite frankly, goldfish just plain have to "be in the mood." You can't force them to breed. They either will or they won't. Take some more time before you decide to do this. But, your goldies are still bunches of cute, lovable fun, even if they aren't mommies and daddies!

Well just by keeping different varieties of goldfish together, you might end up with an accidental breeding and get undesirable results.
I would choose a variety that you like best and breed that variety only to start with.  Then after you learn more about various breeds and genetics of cross breeding then you could start trying to cross breed your own.

Why the heck is this? I've heard about a lot of "black moors" turning into gold telescopes