Can you help me make a decision between node.js/ruby/python for creating a backend for iOS applications that will send push notification and collect data?

try golang

Choose the one you are most familiar with. They can all be used for what you need, and you'll be fastest with the one you already know. If you are not familiar with any of those, then choose the one that looks simplest to use. Really, there isn't that much difference between them and you can do what you need with either of them. However, since you are doing this for iOS, you may be using Swift already. In that case, node.js might be the easiest to learn simply because JavaScript and Swift have somewhat similar syntax, unlike Ruby and Python. I've developed something similar using node.js and it was pretty easy - there are libraries for easy push notification sending, and everything else is just your logic.

We in Countly have examined a few scenarios and end up with using Nodejs for iOS/Android push notifications. If what you are building is not a specific product, I suggest you use our open source backend in Github: http://github.com/countly/countly-server