Heat a large skillet over medium-high heat and melt 1 tablespoon of the butter.
Add the shrimp in a single layer and cook, stirring occasionally, until it just begins to turn pink, about 1–2 minutes.
Stir in the minced garlic and lemon juice, then add the remaining 2 tablespoons butter and the red pepper flakes if using.
Continue cooking, stirring, until the shrimp are fully pink and opaque, about 2–3 more minutes.
Remove from heat, stir in the chopped parsley, adjust seasoning if needed, and serve immediately.