This is often referred to as the central power alliance. You do not have to have a PhD in geometrics to understand this name. Italy, Germany and Austria are the central powers on the board. This alliance is not only a choice; especially for Austria, it is one of the main reasons to survive the game. The advantage of the central power alliance is that it is based on the need for survival, and it does not exclude any other alliances. Assuming that in both the northern and southern triangle two countries will join each other to join the third; it gives you great support in your back playing both Germany and Austria. In 99% of the games, Austria and Germany will minimally agree on a non-aggression pact, so basically the real catch here is that Italy joins their alliance.
The real question is: how do you get Italy to join? A possible answer is to offer him the Lepanto move. This gives him a way forward to attack Turkey and a view on future growth. Another method is to send Italy west, but that should be combined with some support from Germany (perhaps an invasion of Burgundy). Another reason for Italy to join is to instill distrust in Russia in Rome. The main ally in the southern triangle for Italy early in the game is Russia as Turkey will want to control the Mediterranean in the midgame; Italy and Turkey do not really do well in the medium term. However, Russia is a completely different matter. So if Italy sees that Russia is not willing to make a move early towards Austria, Italy will probably not want to do this on his own, especially if a potential German army could show up in Tyrolia for some additional Austrian support.
I often like to have a central power alliance, at least up to 1903. I also believe that Italy is the most obvious contender to quit this alliance, often hinted to do so by Russia or perhaps Turkey. To prevent this from happening will stay difficult and requires sufficient diplomacy from both Germany and Austria.
Germany can be a crucial role in this by contacting Italy as a neutral power. Austria and Italy will be talking quite a lot but both players will have certain distrust in the back of their head because of the proximity of Trieste and Venice. Germany can take in a much more open way with the Italian player. Hence, I believe it is ultimately up to German diplomacy to make this alliance start.