In addition to the modern feel, v3 includes a number of improvements. The Hosted Fields feature automatically formats card numbers and expiration dates which makes for easier text entry and reading. The Hosted Fields functionality has been open sourced. Additionally, file size is greatly reduced because v3 is now organized as a collection of separate components. Instead of consuming a single .js file, users can pick individual components as needed. The smaller file size should lead to lesser impact on load times.
Braintree has updated its docs along with explanations and examples for v3 and all of its APIs. A transition to v3 eases more advanced integrations, allows for quicker load times, provides more flexibility, includes Hosted Fields formatting, and removes the need for forms. Further, Braintree has open sourced v3 as it previously did with iOS and Android versions. Participate in the project at GitHub.