Viniéndonos is the gerund of "venir" (to come), used here in its very explicit sexual sense, meaning reaching orgasm together rather than the everyday meaning of "arriving".
In "al mismo tiempo viniéndonos" ("the two of us cumming at the same time"), the word is blunt and direct, describing an intimate, simultaneous moment between the two people in the song.
Kevin Roldan's "3 AM" drops us right into a late-night, clandestine affair. Our singer is waiting for that call, that text with a location, ready to go because, as he says, "Son las tres de la mañana, esperaba tu llamada / Mándame tu ubicación bebé, voy para allá" [It's three in the morning, I was waiting for your call / Send me your location baby, I'm on my way].
It's clear this isn't a new thing; it's become a habit, a "vicio" [vice] for them to meet up constantly. He knows she's not happy with her current partner, and he's always there to "rescato" [rescue] her. He's a bit blunt, asking why she keeps lying when she knows "conmigo todo es más rico y nos seguimos viendo" [with me everything is better and we keep seeing each other]. The lyrics paint a picture of secret rendezvous, fueled by drinks, ending up in bed together. He even boasts about knowing the "partes más íntimas de tu cuerpo me las aprendí" [most intimate parts of your body I learned them]. It's a pretty direct and confident take on a secret relationship.