when you create an event you can add only once "On Key Pressed", every other condition or sub-event will give only the option "Key is down".
These are two different kinds of events. "On" event is a trigger, it fires only once when the key is pressed. Only one trigger is allowed per event. "Is" event is a condition, if this key is down, the event will run every tick, until the key is released.
So the easiest way to fire the event once all three keys are pressed is this:
On D pressed
Is DownArrow down
Is RightArrow down
But of course it doesn't take into account how fast the keys were pressed and in what order. You need to save the time when each key was pressed. The array seems like an overkill for me, you can do it with a pair of variables - pressedRight and pressedDown.
On DownKey pressed
Set pressedDown to time
On RightKey pressed
Set pressedRight to time
On D pressed
Is DownArrow down
Is RightArrow down
pressedDown > (time-0.5)
pressedRight > pressedDown
....... Play combo animation
This event will fire when all three keys were pressed within 0.5s and in this order: Down->Right->D