If you’re updating from 2.x you’ll need to update some of your template markup as some of the primary tags differently in v3

Main Changes

1. taxonomy:nav tag does not automatically add the <li> elements any more, giving you full control over the output markup.
2. taxonomy:breadcrumbs is now a tag pair, also giving you full control of the output markup.
3. exp:taxonomy:entries is a new tag which largely removes the need for embeds when outputting child teasers on landing pages.
4. No more ‘Use Page Uri’ options in the backend; if a node entry has a uri, Taxonomy is going to assume you want to use it. (That means Structure support out of the box.)
5. The UI has been redesigned and the db schema now uses nested set and adjacency model for heirarchy.
6. exp:taxonomy:get_sibling_ids is now exp:taxonomy:sibling_entry_ids
7. exp:taxonomy:get_children_ids is now exp:taxonomy:child_entry_ids
8. Introduction of a taxonomy_updated extension hook which fires when a tree is updated (allows you to clear caches for example)
9. The taxonomy:nav tag can also act as a single tag, meaning a simple unordered list is generated with the full set of existing parameters available to you

Please review the update instructions if you are updating from Version 2.