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.
If the goal difference was 1 or 2 goals, I would say that Real Madrid have a chance to come back. Because Real Madrid have done this many times in the past. But against Arsenal, the goal difference is 3-0. As we can see from Arsenal's strong defense this season, it is very difficult to score 3 goals against this team.
In the first round, we saw that Arsenal conceded only 3 goals in 8 matches. Although they did not win by a large margin, Arsenal's performance was very stable. In the round of 16, Arsenal won the first leg by a margin of 7-1. The second leg was not that important for Arsenal. Arsenal drew the match 2-2.
Real Madrid fell behind by a large margin in the first leg against such a team. Even though the second leg is a Madrid home match, the chances of Madrid winning by a large margin are very low.