We are using the inhouse Mongo DB instance. We have 3 multisite solution with 3 websites each.
We are having a debate on whether each sitecore instance have their own mongo DB collection databases (reporting, analytic etc) . Or share one for all instances.
Does anyone has experience on this? Any suggestion would be helpful.
On
You can happily do this, as long as you want a single view of your traffic. The reporting mostly allows filtering by Site name, so be sure to call your sites different things. Also avoid the temptation to call all the home pages "Home" and use the site name, e.g. Potato Home, Badger Home, etc. The Page-Level reports show the Node's Display Name and they can be confusing otherwise. Imagine Home, Home, Home and Home all as the highest entry pages.
The xDB processing scales fairly linearly so adding 3 sites should be no more processing than if they were all separate, in total. We always use an external processing server and put a Mongo replica secondary on that box. Then in the Mongo connection string set the ?readPreference=nearest. Check the Mongo docs to see the options you can set on the connection string URL.
It will always better to track analytics data independently for a single instance of a website so as you mentioned there are 3 instances of websites with multisite for each of 3. So as per my recommendation you should go with 3 different xDB databases configuration for each physical instances of websites. It will give you a clear data for the reporting as per Instance wise, if more details you want please do comment.