I think it's random, today I had two 5 dino sessions and one with 6 that included a double dino encounter. However, Saturday I had quite a few 6-7 ones. On two rare occasions, I actually only had 4 and then they went into hiding.
I wish for more triples, too! I have about 2k kills now, including the Three in Bed achievement, and I have only seen maybe 5 triples altogether--of course the majority came before I finally was able to do it, only one after.