3. Please list three most important factors in an ever-lasti

They are Intimacy (亲密感）, Commitment（承诺） and Passion（激情）.
Intimate love felt between two people means that they each feel a sense of high regard for each other. They wish to make each other happy, share with each other, be in communication with each other, help when one is in need.
Commitment, or committed love, is for lovers who are committed to being together for a long period of time. Marriage usually plays the role of commitment.
Couples in passionate love feel physically attracted to each other.Passionate love is not limited to sexual attraction, however. It is a way for couples to express feelings of nurture, dominance, submission, self-actualization, etc.[5] Passionate love is considered the "hot" component of love because of the strong presence of arousal between two people.
The above is from the triangular theory of love (爱情三角理论）
Reference: https://en.wikipedia.org/wiki/Triangular_theory_of_love#cite_note-Levy.2C_P._E._2013_pp._316-317-5
不明白可追问，希望可以帮到你。