On OS X El Capitan 10.11.1, the following terminal commands seem to solve it for me on the Magic Trackpad (on the built in trackpad I can operate only one dragging mechanic at a time) :
# Enable "tap-and-a-half" to drag.
defaults write com.apple.driver.AppleBluetoothMultitouch.trackpad Dragging -int 1
defaults write com.apple.AppleMultitouchTrackpad Dragging -int 1
# Enable 3-finger drag. (Moving with 3 fingers in any window "chrome" moves the window.)
defaults write com.apple.driver.AppleBluetoothMultitouch.trackpad TrackpadThreeFingerDrag -bool true
defaults write com.apple.AppleMultitouchTrackpad TrackpadThreeFingerDrag -bool true
source: https://github.com/boochtek/mac_config/blob/master/trackpad.sh
Update: dug up a bit more and found the "tap behavior" mouse defaults entry, which requires the "2" preset in order to get "tap to drag" on the built in trackpad
defaults -currentHost write NSGlobalDomain com.apple.mouse.tapBehavior -int 2
defaults write NSGlobalDomain com.apple.mouse.tapBehavior -int 2