Not completely sure if this should go in speculation or confirmed, but since its confirmed that starter details come on next weeks episode (not this weeks), and there was a sign with a question mark over it on the last episode, I'm pretty sure this week will give us some shillouettes or a shillouette (or something of a similar nature) of the starters.
A new type triangle seems unlikely because the current types are uncommon for the beginning part of th games, whereas the other type's aren't. Water and fire types don't work well as Pokemon for the first few areas because they don't fit in as well, and also you can't usually surf (or even fish) for a while.
Grass/Ground/Poison
-Poison is common, ground types can usually be found within 1 dungeon and almost always within 2.
Fire/Rock/Steel
-Rocks extremely common, the other 2 are not. Also, I don't see why they would re-use a type if they were going for a totally new type triangle.
Fighting/Rock/Flying
-Fighting is the only uncommon one.