I think I'm somewhere in the teens on this. Some of the vulnerable species don't tend to go to three stars during their lifespan. I had a couple of gold star komodo dragons that popped out several clutches and finally died of old age (and in a zoo with 1.5x slowed animal aging) without getting past two stars. Same with my tortoises, and they live a long, long time.
I've had most luck with my dingos, for some reason. I have to remember to release them right before they become infertile. This is not exactly realistic: I don't think any zoo re-introduces dingoes to the wild, and they certainly wouldn't wait until an animal is nearly senescent to do so, but that's how long it takes for the animals to get to three stars.
I notice the challenge is proceeding pretty slowly at the community level. Seems like this always happens when there's a star requirement for animals. The system for advancing animals popularity with guests (which is what I assume the star rating signifies) is really erratic and random, it seems, especially for species that aren't "super sexy" like lions or giraffes.