Isolation pucks

[octaviars]

Three are always considered better than four as it is impossible for three to rock or wobble. (It's why milking stools always have three legs).

I use Ansuz Darkz D-TC, two in the front and one in the back. As you say one in the back may cause the amp to tillt if the cables pull it down but that is no problem with my amps.
If this is the best position I dont know but my dealer recommended this placment so I went with that.

Now I am confused  

Dont be confused.  

The amp cant wobble with three points of contact even if the shelf is uneven. If you have four it might not put equaly much force on all feets. 

But if you have one foot in the middle at the back of the amp ( two in the front) and for say one really heavy powercable in the left corner at the back pulling down that might cause the amp to tillt down in that corner but likely this wont happen.