Three words

Three words that tie two together.
Three words that make love.
Three words that create harmony.
Three words that make you flutter.
Three words that mean I love you.
Three words that mean I need you.
Three words that mean I want you.
Three words that mean I forgive you.
Three words that make a relationship last.
Three words that no one forgets.
Three words that make you believe in so much.
Three words that last forever.
Three words that brings you joy.

Of all the three words there are how will you know of which to speak?
When there are four phrases too many, which one do you seek?