This 3-in-1 Rounded Corner Punch is from Crafter's Companion.

This punch will help you add rounded corners to your projects that measure 4 mm, 7 mm, and 10 mm. There is a separate slot for each size on the top of the punch, so line that up and then press the lever for a perfectly rounded punch.  This lever applies the perfect amount of pressure to cut detailed designs from cardstock or paper up to 250 gsm.

This punch has a non-slip base, so it will stay in place while you work. This punch also has an extendable base for larger sheets of paper so that you will be all set for lots of punching! This punch measures 3.5" x 3" x 1.8".