1. I like you. Me gustas. Suki-desu -> very early in the relationship, or to start off
2. [blank]. Te quiero. Daisuki-desu -> formal couple, to express feelings of love
2.5 I love you -> formal couple after long time dating I believe, a bit stronger than those two terms above
3. [blank]. Te amo. Aishiteru -> poetic, life-partners, basically we can get married
The "I love you" in English sits somewhere between those two in Spanish/Japanese. If forced to put it in a group, probably the second one would be better and I'd say there's no 3rd one equivalent, since it's really not as strong as the 3rd one, but stronger than the 2nd one.