If it's your first time writing romance, you should focus on a single relationship. It's just easier. If you add a third character in, you triple the number of relationships (as you now have both characters x the MC AND the relationship between the love interests). A fourth character means that there are now 6 different relationships that you need to consider and develop if you don't want the story to stagnate or feel shallow. Those relationships might exist anyways, but they'll be less complicated if they aren't romantic or sexual in nature.