Grilled Potato with tomatoes

2 x Idaho baking potatoes , like russets peeled, sliced     into 12 pcs about 1/2"-thk, and     punched out with round cookie cutter 3 sm purple baking[...]

2 ears of fresh sweet corn, husks and silks removed 2 lbs small red potatoes , washed and cut into 3/4 inch cubes 1/4 cup extra virgin olive oil , plus 3[...]