Look through seaweeds for any small pebbles or debris and discard. Soak wakame and arame in separate bowls with cool water to cover generously until tender, 5 to 12 minutes for wakame and 9 to 10 minutes for arame. Drain well. Snip dulse into 2-in.-long pieces; don't soak.
Toast sesame seeds in a large frying pan over medium heat, stirring often, until golden, 3 to 4 minutes. Pour into a bowl.
Add 1 tablespoons oil to the pan with the garlic and onions; sauté over medium heat until softened, 1 minute. Remove from heat.
Add seaweeds, remaining 1 tablespoons oil, and soy sauce. Toss gently to coat well and let stand until dulse is softened, 5 minutes. Mix in sesame seeds.
Taste and add rice vinegar if you like. Serve warm or at room temperature.
*Buy dried seaweed at natural foods stores and Asian markets.