1. Turn off a setting
Go to the Trackpad pane in System Preferences and click its More Gestures tab. If the gesture for Mission Control (Swipe up with three/four fingers) has a check mark next to it, clear it.
Return to BetterTouchTool and add a new gesture.
2. Define your override
In the Touchpad Gesture pop-up, open the Three Finger Gestures group and select 3 Finger Swipe Up. In the Predefined Action pop-up, expand OS X Functionality and choose Mission Control & Immediately Show Desktop Preview.
3. Try Mission Control
Swipe three fingers upwards and Mission Control will open, and the Spaces bar will expand to show desktop previews after a brief pause; El Capitan normally requires you to move the pointer to the top of the screen to display them.
