How to: 3 fingers tap to show Activities (Gnome Shell) ?

[Frédéric Mesplède <fredericmesplede@gmail.com>]

Hi everyone!

I read the Debian wiki about Synaptics (
https://wiki.debian.org/fr/SynapticsTouchpad ) but I can't figure out
how to activate 3 fingers tap... I would like to create a sort of
shortcut to show Activities whenever I tap three fingers on my
touchpad. Any ideas?

My computer is:
Debian 8 Stable Gnome 64bits
LDLC Aurore BB5-I3-8-S1 Slim (Clevo W55xEU)
Intel Core i3-3120M / Intel HD Graphics 4000 / RAM 8 Go / Samsung SSD 840 120Go

frederic@frederic-laptop:~$ grep "TouchPad: buttons:" /var/log/Xorg.0.log
[     3.388] (--) synaptics: SynPS/2 Synaptics TouchPad: buttons: left
right double triple

frederic@frederic-laptop:~$ synclient -l
Parameter settings:
    LeftEdge                = 1767
    RightEdge               = 5397
    TopEdge                 = 1637
    BottomEdge              = 4451
    FingerLow               = 25
    FingerHigh              = 30
    MaxTapTime              = 180
    MaxTapMove              = 234
    MaxDoubleTapTime        = 180
    SingleTapTimeout        = 180
    ClickTime               = 100
    EmulateMidButtonTime    = 75
    EmulateTwoFingerMinZ    = 282
    EmulateTwoFingerMinW    = 7
    VertScrollDelta         = 106
    HorizScrollDelta        = 106
    VertEdgeScroll          = 0
    HorizEdgeScroll         = 0
    CornerCoasting          = 0
    VertTwoFingerScroll     = 1
    HorizTwoFingerScroll    = 1
    MinSpeed                = 1
    MaxSpeed                = 1.75
    AccelFactor             = 0.0374602
    TouchpadOff             = 2
    LockedDrags             = 0
    LockedDragTimeout       = 5000
    RTCornerButton          = 0
    RBCornerButton          = 0
    LTCornerButton          = 0
    LBCornerButton          = 0
    TapButton1              = 1
    TapButton2              = 3
    TapButton3              = 2
    ClickFinger1            = 1
    ClickFinger2            = 1
    ClickFinger3            = 1
    CircularScrolling       = 0
    CircScrollDelta         = 0.1
    CircScrollTrigger       = 0
    CircularPad             = 0
    PalmDetect              = 0
    PalmMinWidth            = 10
    PalmMinZ                = 200
    CoastingSpeed           = 20
    CoastingFriction        = 50
    PressureMotionMinZ      = 30
    PressureMotionMaxZ      = 160
    PressureMotionMinFactor = 1
    PressureMotionMaxFactor = 1
    GrabEventDevice         = 0
    TapAndDragGesture       = 1
    AreaLeftEdge            = 0
    AreaRightEdge           = 0
    AreaTopEdge             = 0
    AreaBottomEdge          = 0
    HorizHysteresis         = 26
    VertHysteresis          = 26
    ClickPad                = 0

So, I did the first part of the job. I think I enabled the three
fingers tap by creating this file /etc/X11/xorg.conf.d/synaptics.conf

Section "InputClass"
        Identifier      "Touchpad"                      # required
        MatchIsTouchpad "yes"                           # required
        Driver          "synaptics"                     # required
        Option          "TapButton1"            "1"
        Option          "TapButton2"            "2"     # multitouch
        Option          "TapButton3"            "3"     # multitouch
        Option          "VertTwoFingerScroll"   "1"     # multitouch
        Option          "HorizTwoFingerScroll"  "1"     # multitouch
EndSection


Now, how to link it to showing Activities?

Thank you for your help!