Infierno literally means "hell." While it's a very strong word, it's often used in Spanish songs and poetry as a powerful metaphor for emotional suffering.
In this song, Enrique Iglesias expresses his desperation by asking how he can "escaparme de este infierno" (escape from this hell). This dramatic use of the word vividly paints a picture of his torment and longing, making it a very memorable line.
Ever been so completely tongue-tied around someone that your brain just stalls out? That is the exact panic Spanish singer Enrique Iglesias builds in "Tres Palabras". He is losing his mind over somebody, claiming there isn't a single quiet moment where they aren't missing: "No hay un silencio en que no faltes" ("There isn't a silence where you aren't missed"). He has three specific words stuck in his throat—which he never spells out, though te quiero or te amo are pretty safe bets—and every time he looks at this person, the phrase evaporates: "Son tres palabras que no sé cómo decir / Y que se pierden al mirarte" ("They are three words I don't know how to say / And they get lost when looking at you").
There is a great everyday Spanish idiom hidden in the insomnia he describes: "La noche pasa en blanco" ("The night passes in white"). In English, we say we spent a "sleepless night" or "pulled an all-nighter," but in Spanish, passing the night en blanco means staring at the ceiling until dawn without getting a wink of sleep. His chest is practically rattling because his heart refuses to shut up—"No quiere callar mi corazón" ("My heart does not want to keep quiet")—panicking that love might vanish as quickly as it arrived ("Llega de pronto y huye sin decir adiós"). It is dramatic, sweaty-palmed romantic agony at its most honest.