I don't really think if Real Madrid can be able to comeback in the second leg. Because the three goals margin can never be that easy for Real Madrid to level without Arsenal scoring more goals in the game. The three goals defeat is a very big advantage to Arsenal because they are more balance right now in this second leg than Real Madrid, all they just need now is to only hold Real Madrid draw in this game.
Thinking that Madrid will be able to come back is like Thinking that Arsenal will not be playing in this game, I know anything is really possible in football but there are situations that when you look at critically and consider that which is involved you just understand and conclude that certain things are very likely not to happen such as this, Really Madrid need 4 goals to have a proper comeback enough to get them knocking Arsenal out and they need all four goals in one match but again Arsenal has got a better defence than Madrid does so I'm wondering how the magic will be done except if Arsenal gets to suffer red cards or have their key player injured which is almost not happening so it's an almost not possible task for Madrid but let's see, I will never say never because it's not over until it's actually over.
Like you said it's football and so many things can happen so don't actually write off the possibility moreover it's not the first time this kind of thing has happened so am very optimistic to see how this return leg plays out and again Real Madrid doesn't actually necessarily need four goals to make a comeback because they can play the game 3-0 in favor of them and drag it to extra time and maybe penalty shoot out.