#1
It looks like he's holding it between his thumb, middle- and index finger. It looks like he's using 3 fingers but I'm not sure. Does it have any advantages/disadvantages by playing the guitar this way?

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#2
I know he has a thing about the number three, but using three fingers to hold a pick sounds a little bit overkill.

In seriousness though, I'm not really too sure, but how you hold your pick, and the type of pick you use, can affect the tone to an extent - some positions give a slightly more percussive sound, while others allow you to pick a little bit faster. I wouldn't go trying to emulate his style too much though, just choose whatever is most comfortable for you. 
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#3
donender Thank you for your answer. I was curious and tried it myself. On the one hand it was more comfortable to play the the 3 high strings. On the other hand hitting the bass strings while palm muting got a bit more difficult.
#4
Quote by MoaninE
donender Thank you for your answer. I was curious and tried it myself. On the one hand it was more comfortable to play the the 3 high strings. On the other hand hitting the bass strings while palm muting got a bit more difficult.

Well as I said, go for a picking technique that suits you. You wanna be comfortable while playing, and if you go for something that doesn't feel natural, you won't be able to play more than three songs before your hand or arm starts to hurt, which is no fun.
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#6
Some people hold it that way, but it's generally not advisable. It doesn't change the sound in any way, and limits the use of other fingers. Nothing about Jack White's sound is related to that pick grip.
#7
Quote by MoaninE
donenderHey, finally I found a good picture showing him holding the pick. He's actually holding it with 3 fingers. I wonder if it's, as you said, because of his obsession with the number 3.

Here's the picture: https://ksassets.timeincuk.net/wp/uploads/sites/55/2012/07/7535240976_b46e875d66_b-1.jpg

Yeah see, that looks awkward to me. It may work for him, but as cdgraves said, that's one of the more restrictive ways of holding a pick, and his sound doesn't really come from the three fingered pick grip - it's more about his guitars, amps and pedals. 
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