Is the damage of the attack that triggers Press the Attack also amplified?

Solution 1:

As of patch 7.24 all bonuses apply on and after the 4th hit

So I just tested this in the practice tool and the 12% bonus damage are applied on the 3rd hit for both the initial attack and the bonus effect.

The 12% damage taken increase will be applied before the damage.

For testing I used Siphoning strike (about 1.4k stacks) and Sheen on Nasus and I've also tested Kayle's E. Other attacks and effects should behave the same way.

Solution 2:

An update to Jutschge's answer:

Since patch 7.24, Press the Attack no longer applies the damage amplification to the 3rd hit, but rather after the 3rd hit.