I saw this on a youtube video. The answer is that the one in the middle is carrying 25 % more than the other two.
Can someone explain this?
There is no way to know, just from the picture, because the problem of body equilibrium with 3 upward forces is underdetermined. The middle man could choose to be slacking and pushing only a little, while the two other men carry most of the weight. Or he could choose to carry the load himself, so the other two men will not have to push up at all.
In case you're thinking "they are not pushing, the log is just put on their shoulders" then the problem is still underdetermined; the actual forces will depend on the details of the shape of the log, and the way the log and the bodies of the men deform.