抛物线 y=

1

2

x2 的图象向左平移3个单位，所得抛物线的解析式为（      ）．

A：$y=\frac{1}{2}{x}^2-3$

B：$y=\frac{1}{2}{\left(x-3\right)}^2$

C：$y=\frac{1}{2}{x}^2+3$

D：$y=\frac{1}{2}{\left(x+3\right)}^2$