Throttling problem - why am I getting 429?

I am unaware of  a limitation of 3 requests per second.

Throttling is per minute: http://ringcentral-api-docs.readthedocs.io/en/latest/rate_limits/

But if you do encounter 429, you need to slow down the api access. Best practice is to write proper code to handle 429 instead of assuming you will never encounter it.

And by the way, there might be multiple instances of your app running so they will share the throttling quota.