Addition With Regrouping Worksheets for 3rd Graders Add within 1000 with Regrouping Worksheets for 3rd Graders (20) View all 20 worksheets. Regroup ones and add. Add 3-Digit and 1-Digit Numbers with Regrouping: Horizontal Addition. Mastery in addition strategies within 1000 is now super easy! 2nd graders can thoroughly practice in the add using multiples of 100 with this worksheet.