Three.

The three hardest, three-word sentences you'll ever have to say... when they're sincere. 

"I am sorry."
We say it every day when we run into someone while walking. We say it when interrupting a conversation. We say it in place of, "excuse me." 
That's when it's habitual. When it's hard is when you mean it.
This is admitting you're wrong. This damage is done physically, verbally and emotionally. Maybe it was severe in your eyes, but a misjudgment in theirs. Maybe it was a secret revealed. Maybe it was minor in their eyes, yet catastrophic in yours. Our emotions as humans are different, therefore, so are our reactions.
When you say I'm sorry, it doesn't grant automatic forgiveness. But when you mean it, it gives automatic relief. That being said, we should not apologize simply to free our conscience. Only when we are truly sorry.  

"I forgive you."
This is admitting you were hurt. This is allowing room to breathe in your heart and mind again. We should not plague our bodies with ill memories. If a murderer can be forgiven, so can foul words and actions against us. Forgiveness isn't immediate, but it is an option. And if you forgive, but do not forget, then you haven't truly forgiven. To forgive is to let it go. All of it: the feelings, the thoughts and the reactions. It isn't easy, but carrying it around has heavier consequences. When we forgive, we change. 

"I love you."
Those three irrevocable, unexplainable, magnificent words. We say it to our family and friends habitually. We say it in exaggerated thanks. We say it to pets. And we say it in reference to tangible objects. Love, however, in its truest, is intangible. It's in a look. In a feeling. In a moment. In a lifetime. 
Saying "I love you,' means letting go of the past, letting go of fear, letting go of prejudice, letting go of pride, letting go of the future. Saying "I love you" is the most relieving three words we'll ever say after, "I forgive you" and "I am sorry." Saying "I love you" is living in the now. 
If later, you find yourself saying you don't love someone anymore due to ill circumstances, then, like forgiveness, you never loved them at all. Love is forgiveness. If you truly loved, you truly always will. This is why loss hurts so deeply.