Thermoplastic resin composition, method of preparing the same, and molded article including the same

A method of preparing a thermoplastic resin composition, the method comprising kneading and extruding, under conditions of 200 to 270° C. and 200 to 300 rpm, 65 to 85% by weight of a heat-resistant copolymer (A) containing 76 to 92% by weight of a (meth)acrylic acid alkyl ester compound, 1 to 8% by weight of an N-substituted maleimide compound, and 5 to 16% by weight of an aromatic vinyl compound;

• 10 to 30% by weight of an acrylate-based rubber-aromatic vinyl compound-vinyl cyanide compound graft copolymer (B) containing acrylate-based rubber having an average particle diameter of 50 to 200 nm;
• and 1 to 15% by weight of an acrylate-based rubber-aromatic vinyl compound-vinyl cyanide compound graft copolymer (C) containing acrylate-based rubber having an average particle diameter of 300 to 600 nm,
• wherein, in the heat-resistant copolymer (A), a weight ratio of the aromatic vinyl compound to the N-substituted maleimide compound is 1.5 to 8.