I was just thinking this is gold AH. The men did not live within five miles of each other, and Trotsky's account of meeting Stalin in an apartment of a friend/turned rival may not be true, but in January, 1913, Trotsky, Stalin and Hitler were all in Vienna proper. I was thinking of doing something with this, but the butterflies... there are so many. I think Freud would be ASB to meet the trio. And Tito could have, but it'd be quite hard, so I would prefer to exclude him. But Trotsky and Hitler is a possibility (they frequented the same cafe, though, so did a lot of people, as it was a busy cafe), and Trotsky and Stalin is a definite possibility, or did indeed happen. It is not out of the realm of reason to put all three in a room.
At the time, Trotsky might be interested in meeting Stalin just out of sheer curiosity, IIRC, almost every Georgian Social Democrat was a Menshevik. Stalin being both a Georgian and a Bolshevik is what made him appealing to Lenin, IIRC. Trotsky might have had a chat with the curio as well, though knowing Trotsky, he'd be done with him in five mins or less and move on. There is no way Stalin's conversation skills would keep a busy mind such as Trotsky interested. At 1913 Trotsky was an arrogant man with much to be arrogant about.
What Trotsky would have made of Hitler would depend on which Hitler he was meeting at the time. I am trying to remember in what state of mind, dress and appearance Hitler was in January, 1913 in Vienna. There was a period where he stopped giving a shit about everything, grew out his hair, had a beard and walked around in filthy garish leather lederhosen for months. There were also sporadic bursts of activity, when he would get odd jobs, some of vaguely artistic variety and try to better himself, before falling into a lethargy and getting into arguments with roommates. 1913 Hitler was a quilt of crazy (ya know as opposed to the model of sanity later). Trotsky spoke German. How well depends on who is telling the story, what are their political beliefs and when they are telling the story. To begin with, Trotsky himself said he did not speak any language fluently but Russian and Ukrainian, but he did speak English, German and French. His adherents suggest he was fluent in French. German is more tricky, but he was living in Vienna for a while and being bright and able to pick up things, he would have gotten some knowledge. There is also a barely TMZ worthy evidence he might have spoken Yiddish, due to his surname, but if I had to guess, he did not live in the part of the country where Yiddish was truly spoken and his mom and dad would have likely spoken a mixture of Russian and Ukrainian. However, he did live in Odessa for a time, and ladies and gentlemen, you would have to work very hard at not picking up Yiddish if you lived in Odessa for any length of time back then, or live there now, or will live there in the conceivable future. Odessa is to Yiddish as the Dune sandworms are to spice. Would a bit of conversational Yiddish help his German? Not sure. Regardless, it is plausible for Trotsky to be able to at least have some kind of basic conversation with Hitler if the two were to share a table. The trick is getting Trotsky and Hitler at the same table, and that would depend on the condition of Hitler and Trotsky's mood at the time.
But having Stalin, Trotsky and Hitler at the same table at the same time and talking to each other would be quite a feat. For starters Stalin did not speak any language but Georgian and Russian, and Hitler would not have understood either. So he's not joining in the conversation, unless Trotsky runs translation and that only happens if Trotsky is really feeling it that day and the conversation is amazing enough to require it. I cannot think of such a scenario, but I can picture Hitler sitting in the corner booth (table), Trotsky and Stalin come inside and Trotsky waves to a curio he spoke with before, and the curio acknowledges his presence and Trotsky leads Stalin over to the corner booth (table) and the three have some cakes and tea.