Chamita is a colloquial term, mainly used in Venezuela, for a young girl or girlfriend. It's an affectionate, informal way to address someone close, similar to calling someone 'babe' or 'sweetheart' in English.
In the song, the line "EscĂșchala, chamita" ("Listen to it, babe") shows the singer speaking tenderly and casually to someone dear, inviting her to pay attention in a warm, familiar tone.
"3AM" sings about a love that keeps pulling him back, even in dreams. The song opens with a dream where he sees the person again after a long time apart, and that memory changes everything for him.
He admits he is used to losing, but refuses to accept losing this person. His friends tell him to move on and forget her, but he cannot let go of a connection this deep. He wonders why the people we love the most are often the ones who leave, and why love does not just last forever.
The song's title, "Vidas Pasadas" ("Past Lives"), points to its closing idea: he believes what they have goes beyond this one lifetime. Whether in this life or the next, he still wants her to stay.