I'm not sure about traits but there are two boys and one girl. The game named them Lady, Eddy, and Baxter. They are all identical to the mother, visually at least. I didn't adopt them. I'm on my laptop now, but when I'm on the game again I'll check and report back!
And yes, I was very surprised. I was like, three?! Holy crap! Because I know that twin foals are very unlikely in real life as it is dangerous for the mother, and triplets are really unheard of as far as I know... Also my couple also had another infant that surprised me. Needless to say, I will have my hands full (4 Sims, now 6 horses).