Discount for specific user group in Drupal Commerce I need to create 3 user groups and all of these groups must to have discount on all products in percent.

Example:

Group1 have discount of 10% on all products,
Group2 have discount of 13% on all products,
Group2 have discount of 16% on all products

Any idea how to do that?
Wait, wait, wait. That video tutorial referenced in the currently accepted answer is very outdated and unnecessary. All you need to do is create a new product pricing rule and use the "Multiply the unit price by some amount" line item action. Multiply it by .9 for a 10% discount, .87 for a 13%, and .84 for a 16% discount. Then just select the "Discount" price component (or else use a module to define a new price component type) in the action so it appears as expected in order total price breakdowns.

The only reason abusing the tax system was recommended at the time of that video recording was because you could not specify the price component type in the rules action. That has long since been changed, but alas the video still gets linked to quite a bit.