Does Apple Watch Series 3 have a run walk run feature?

No, it won't.

The option I have described is for manually switching between workout types as and when you wish to do so.

You may find that a third-party interval timer app for Apple Watch is better suited for your needs.

Apps that you may wish to consider include, for example:

Intervals - Workout Interval Timer on the App Store

• "Apple Watch - The most advanced Interval timer for the Apple Watch"

Runmeter Running & Cycling GPS on the App Store

Hi

No, the Workout app does not include a specific run walk run feature.

However, you can record a run walk run workout by starting with an Outdoor Run / Indoor Run and then switching activity types first to Walk and then back to Run again, without ending the session until you have finished the final element.

Once your current workout has been in progress for long enough to be ended and saved as a completed workout (a couple of minutes or so), swipe from left to right across screen within the Workout app and tap on the "+" button to switch to a new workout activity type.

More information:

Work out with your Apple Watch - Apple Support

If you would like to submit a feature request to Apple for more specific support for that type of workout, then you can do so here:

Feedback - Apple Watch - Apple

You may also find that a third-party fitness tracking app offers features that meet your needs. To browse and search for apps with the results pre-filtered to include only those iPhone apps that also include an app for Apple Watch, use the Watch app on your iPhone. Apps can be searched by keyword or app name via the Search tab.

More information

Use apps on your Apple Watch - Apple Support