Preheat the grill to medium. In a small bowl, combine 2 tablespoons of the oil, vinegar, honey, mustard, red pepper flakes, and salt and pepper.
On a sheet pan, toss the asparagus with the remaining tablespoon of the olive oil. Transfer to the grill with the spears facing perpendicular to the grates. Use tongs to turn them as they get grill marks on the bottom, until they are cooked to your liking, anywhere from 4 to 8 minutes, depending on the thickness of the asparagus and how well you like them cooked. Don’t forget that they will continue to cook a bit after you remove them from the heat.
Transfer them to a serving platter and drizzle over the vinaigrette. Serve warm or at room temperature.