This guide will show you where the hardmode button is and the location of the 3 coins.
The button is located above the start area:
Just press it and hardmode will be enable.
The Coins Locations
The first coin is at the "zombie" like room, there is one door that doesn't open automatically you'll need to open it the coin is in it once you are done with the event.
Before the event:
After the event:
The second coin is located after going up a long ladder where the awp enemie is its on the edge:
The last coin is located at the helipad on the higest "place":