12.04 : Three finger gestures disappeared from touchpad?

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http://askubuntu.com – I've recently upgraded from 11.10 to 12.04, and I realize now that I have lost the ability to simulate a middle-click by using my touchpad with three fingers. My laptop is a recent Sony VAIO VPCSE (I don't have the precise reference right here). Is there something to do for me to regain this feature? Thanks in advance. (HowTos)