Threesomes aren't the be all and end all of more-than-two sex, foursomes and moresomes are also common in some fetish clubs and sex-positive social circles, and sex involving four or more people is often referred to as 'group sex'. There's more than one form of group sex - there's the gangbang, for example, where a group of people are all focused on one individual, and there's the orgy, which is more of a free-for-all. People of all and any sexualities and gender identities can enjoy group sex in the right context, and it's a great way to live out some of your deeper and darker fantasies.
Safer sex is even more important in group situations than it is in most others, as there's a far higher chance of picking up something you didn't want when you have more partners at once. You should also have consent negotiations before, during and after--it's vital to make sure that everyone is happy, comfortable and on the same page.
The 'classic' gangbang situation is one woman being fucked by a group of men at once. It's likely to involve double or triple penetration and sometimes ends up with some pretty intense cumshots. There are plenty of variations on the theme, of course, people of all sexualities and gender identities can enjoy a good gangbang - but the core concept is a whole group of people all concentrating on having their way with a single sub. Many BDSM fans find that this is a common and highly enticing fantasy, particularly if they're into the idea of sexual humiliation.
A little bit of scratching is extremely common during sex, be it as a way to heighten sexual tension or an unconscious reaction to extreme pleasure. A lot of people find it extremely erotic, and some have a remarkably strong reaction to the right kind of scratching in just the right spot! As with all things - especially sensation play and anything involving potential pain - it's important to check in with your partner rather than risk doing something they dislike.
For some people, scratching forms a part of BDSM and kink. There are a few relatively common toys designed to assist or enhance the feeling, like the pinwheel or the vampire glove. If you think you might be interested, however, it's a good idea to start with fingernails and work up. You might be surprised by how intense the sensation can be without any external assistance at all.