How to Enable the Three Finger Drag Gesture on Mac Trackpads in OS X
My new iMac came with a Magic Trackpad2 and for some reason the 3 finger drag feature was not there. Here’s how to get it back:

Enabling Three Finger Drag on Mac Trackpads in OS X El Capitan
In OS X 10.10.x and OS X 10.11.x and newer you’ll find the option under Accessibility as follows:
- Open “System Preferences” from the Apple menu
- Choose the “Accessibility” preference panel
- Scroll down the list on the left side and select “Mouse & Trackpad”
- Click the “Trackpad Options” button
- Check the box next to “Enable dragging” and pull down the menu alongside to select “three finger drag”, then click “OK” to set the preference
- Close out of System Preferences as usual and try the feature out
Source: How to Enable the Three Finger Drag Gesture on Mac Trackpads in OS X