I have a feeling they'll all have secondary types, the Johto starters tend to get looked down upon due to their pure typings even in final form (I still love 'em anyway), and I've always thought it makes sense for them to have unusual dual-typings since starters are supposed to be rare Pokemon.
For Tsutarja... I could see him going a lot of ways. Flying would be quite interesting, we haven't had a part-Flying starter since Charizard, and the only other Grass/Flying is Tropius, who's more for HMs than anything. Dragon is a possibility, at first I found it unlikely due to shaking up the triangle dynamics, but then again, Empoleon's Steel-type... the Grass/Dragon potential sounds good to me, I have to admit. I've heard Electric but it really makes no sense to me to say that based on a bit of yellow alone, and Poison's been done to death as a secondary type for Grass. I would say that if any was most likely to stay single-type, it'd be this guy, but we have Meganium and Sceptile already, so I think that's very improbable.
In Pokabu's case... Dark seems most likely to me. Not because he's partially black (see Tsutarja/yellow/Electric), but because if he seems likely to evolve into a boar. With those big tusks, I could definitely see some moves like Crunch or Night Slash... it would probably learn Fire Fang, and a lot of Pokemon that can learn that also learn Crunch, so it seems a logical progression. Ground seems next most likely to me, but we had a part-Ground last generation so I hope they try a new type. I can't really see any other combos as particularly likely.
Finally, Mijumaru... he's the toughest for me to say, I could see him as Ice or Rock. A part-Ice otter seems acceptable, and it would counter Tsutarja (especially if he becomes a Dragon). On the other hand, Rock is possible, a lot of the potential evos I've seen play up the shell aspect, and I hear he has a move called "Shell Blade" or something to that effect. After Omastar, Kabutops, Corsola, and Relicanth, I do think it's a very nice but underappreciated type combination. I just can't say something very definitive for some reason, he's such an unusual starter that he could be a potential wildcard.
I have high hopes overall for these starters, though. September's too long to wait.