According to the Microsoft changelog on the Surface Pro 3 Update History support page, there is the complete list of changes:

Surface Pro Embedded Controller Firmware: improves system stability and performance when you change between power states.

The new April 2017 firmware update for Surface Pro 3 should download and install automatically. However, if you have not received the update on your Surface, then go to Settings > Update & security > Windows Update and try to force the download manually. All content on this site is provided with no warranties, express or implied. Use any information at your own risk. Always backup of your device and files before making any changes. Privacy policy info.