Alik, you are correct in that the ids have changed in v3. There is now only one id for all content types, which makes life easier in most situations.
Jive is still using the old id convention in some areas of the v3 api. So to address this, and if you want to use the v2 convention, you can do so in the following format:
if you use this, you can get the v3 id using resources.self
This will retrieve the document with the ID 5150 using the v3 api, but with the v2 id convention. You do need to know the contenttype id, 102 is for documents, here is the complete list: https://community.jivesoftware.com/docs/DOC-7329