A) Their roles in such a world aren't that different from one another, and
B) that role is a little boring
Their similarities are why I decided to lump them into one entry. So we'll talk about those first.
First, the stats.
Att: 1 (tail or trample)
Move: 60' (20')
No. App: 0 (1d4)
Att: 1 (butt or trample)
Move: 120' (40')
No. App: 0 (1d6)
Att: 1 (gore or trample)
Move: 90' (30')
No. App: 0 (1d4)
Each of these are large plant-eaters, but not so large as the mega-herbivores like Apatosaurus. Don't get me wrong, they're big. But the aren't HUGE.
Each of these animals has decent HD. they aren't particularly fast, either. They can also deal out a reasonable amount of damage, but they have skittish morale. They aren't after a fight. They are generally prey, not predators. They are herd animals, though. granted the number appearing stats are pretty small herds, but it stills add up to a lot of prehistoric trampling and goring on the
Now let's talk about some differences.
You'd think the Titanothere to be the most dangerous in general, due the extra HD and the slightly faster movement. Not to mention the higher damage. It isn't as well armored as the dinos, but it does have a bit of hide to get through.
No, my money is on the triceratops. The two things that make me vote that way come from the description.
"Although these creatures are plant eaters, they are aggressive and dangerous, usually attacking on sight. They charge for double damage on the first attack."So they are more likely to pick a fight, (slightly) less likely to run away, and their initial attack can do up to 36 points of damage. To be fair old Tricorn is a bit slow, so even on the charge he isn't running down able-bodied PCs, but you do to get caught walking when its initiative comes up.
Lastly, if I were actually running a lost world setting with all these creatures from various eras and epochs mashed together, I'd have one word about these prehistoric herd animals.