Each update incorporates any fixes and features from the previous updates. You can find them here:
Each update, the Maya Help includes new feature documentation. Revisions include clarifications and corrections that are the direct result of feedback we receive from the Was this article helpful? space at the bottom of each page in the online help. This tool lets us know how we can improve the help to better serve your needs.
If you do not upgrade your software with each update, you may see help topics where information has changed or does not apply to your version of Maya. This applies to Maya technical documentation, API Guide, Maya User Guide, and other resources.
For information regarding new API features in Maya 2017 Update 3, see What's New in API in Maya Update 3.