Consider the puzzle to the right. Note that it has much fewer given numbers at the start (sometimes only one), so you will always need to apply frame sum techniques at the beginning of each puzzle.
The easiest cells to solve is when two out of three cells in a triplet are known. In this puzzle we see that the triplet in column 9, block 2 has two cells already known. We know that the first triplet in column 9 sums to 15 and the last to 19 and all the cells in the column always sum to 45 (1+2+3+4+5+6+7+8+9) - therefore the middle triplet must sum to 11. So the highlighted cell must be 1.