Repetition is the most obvious literary technique used by Pablo Neruda in his poem "Tonight I can Write." The line "Tonight I can write the saddest lines" is repeated thrice in the poem  to emphatically foreground his sense of loss and sadness and to express his feelings of loneliness after the woman whom he loved has left him, "Tonight I can write the saddest lines/To think that I do not have her. To feel that I have lost her." 

Throughout the entire poem he struggles to come to terms with the fact that the woman whom he loved deeply and passionately is no longer his but, "will be another's."  This particular night reminds him of his  passionate encounters with her in the past, only to reiterate the fact that she is no longer his: "Because through nights like this one I held her in my arms/my soul is not satisfied that it has lost her." And so, to highlight his sense of loss and loneliness he repeats the line "Tonight I can write the saddest lines" thrice.