I've been to goa 6 times till now, and all the times I've been in a relationship , so I do not have any personal stories of having threesomes.
But I do know a friend of mine who had a threesome experience
Their story is this:
They went to a very famous clubs near calangute beach, once inside the club the couple got very drunk and the girl started making out with other random girls, and soon enough one of the girls was very interested to join in to take things to the next level with the couple and the rest of the story I'll leave it up to you.
It's very rare that I've heard of such stories, of people randomly hooking up. But they do happen, there's nothing taboo about it in goa.
Couples( including me) get invited to many "off the scene" parties were people party and some couples swap or have a foursome back in their rooms, these are more common rather than threesomes.
As for nudity on the beaches, it depends on the time and place. The first time I went, I had seen a lot of topless women(no one completely in the buff however) and completely nude men strolling on the beaches. However on my subsequent visits, the number of nude men and women have become almost none. You can see women sunbathing topless but with their backs to the sky.
As for personal experience, my girl friend at that time, who was comfortable wearing a bikini was not comfortable walking topless on the beach during the day. During the nights however on beaches near ashwim, lets just say, sex on the beach wasn't a coctail.
As for drugs, it's unimaginably easy to get around there, ask any shack owner and they'll give you a pack of cocaine, LSD and marihuana. I haven't heard of any other drugs that are peddled by them. Only precaution that you should exercise is not to openly flaunt your drugs in front of the cops.