Press the Attack is a new rune during this preseason's rune changes. It's effect causes enemies to take 12% more damage from all sources if you basic attack them 3 times.
Some champions have abilities that enhance their auto attack damage. For example, Vayne's passive does extra true damage if she attacks the same target three times. Ezreal's Q applies on-hit effects, so it's (kind of) a skill shot basic attack. Nasus' Q does more damage the more stacks he has. Etc. etc.
Does the extra damage from these "enhancements" get amplified by 12% on the third attack, or does that boost only start after Press the Attack has been triggered (fourth attack and onward)?