\[3x^{5}y^{2} + 15x^{4}y^{3} + 12x^{3}y^{4} =\]
\[= 3x^{3}y^{2}\left( x^{5 - 3} + \frac{15}{3}x^{4 - 3}y^{3 - 2} + \frac{12}{3}x^{4 - 2} \right) =\]
\[= 3x^{3}y^{2}\left( x^{2} + 5xy + 4y^{2} \right).\]