Add 3 cloves garlic, juice from 1 medium-large lemon, 1 teaspoon salt, and ½ cup neutral oil to large mixing bowl. Whisk ingredients together until well incorporated. Set bowl aside.
Wash 18 medium red, white, and blue potatoes, making sure to completely remove dirt from potato skins. Once potatoes are clean, slice each potato in half, then add potato halves to bowl with oil mixture.
Toss potatoes to fully coat in oil mixture, then cover bowl and marinate potatoes at least 15 minutes, up to 1 hour.
Toward end of marinating time, begin preheating grill to medium heat, approximately 350° to 375° Fahrenheit.
After marinating potatoes for desired length of time, uncover bowl and thread one red potato half, one white potato half, and one blue potato half onto each skewer. Position potatoes so that all 3 face same direction.
Once grill is preheated, place potato skewers directly onto grill grates over indirect heat, cut-side down.
Close lid and grill potatoes 15 to 20 minutes or until potatoes are fork-tender. Before removing potatoes from grill, flip skewers over and grill potatoes skin-side down 3 minutes.
Remove potato skewers from grill and let cool slightly. Serve warm with ranch dressing.