Well, first thing's first, Pokemon Go is based completely on a random-selective system. Think of Pokemon Go as a regular Pokemon game, some Pokemon are much rarer than the others. The only difference, Pokemon Go doesn't have a particular spot to catch a certain Pokemon - unlike Pokemon MMO or other device games. The chances of finding any Pokemon in Pokemon Go can literally be found anywhere.
(It all depends on release schedules in concern to legendaries)
That being said this article can help you see the probability of encountering such a rare Pokemon as those 3 starters. Chikorita, Cyndaquil, and Totodile are all located under/considered as very rare, so your chances of finding them are not going to be quick and easy.
So, to answer your question, eventhough I haven't caught a Cyndaquil in Pokemon Go, it has approximately a 1-5% of being encountered anywhere in the game - and the creators did say starters are one of the most challenging Pokemon to find.
Hope this helps! :)