Prepare a grill for direct high heat (450° to 550°; you can hold your hand 5 in. above cooking grate only 2 to 4 seconds). Rub corn with 1 tbsp. oil. Lay ears on cooking grate (close lid if using gas). Grill corn, turning occasionally, until lightly browned all over, 8 to 10 minutes total. Remove and let cool. Holding ears upright in a deep bowl, cut kernels from cobs with a large knife.
Meanwhile, in a saucepan, bring broth to a simmer. Pour remaining 2 tbsp. oil into another, large saucepan over medium-high heat. Add shallots and cook, stirring often, just until softened, 2 to 3 minutes. Add rice and cook, stirring often, until grains are slightly translucent at edges, about 3 minutes longer.
Add wine and cook, stirring often, until almost absorbed. Add broth one ladleful at a time, cooking and stirring until almost absorbed, before adding another. Continue cooking and adding broth until rice is barely tender and mixture is creamy, about 25 minutes. (You may not need all the broth.)
Stir in corn, shrimp, butter, tarragon, lemon zest, lemon juice, pepper, and salt to taste and cook, stirring often, until heated through, about 3 minutes. Add more broth if risotto gets too dry.
Spoon risotto into wide, shallow bowls.
Note: Nutritional analysis is per serving.