对于有有理数$a$,$b$定义一种新运算"$\Delta$",规定$a\Delta b=-2b+3a$.
$(1)$计算:$\left(-3\right)\Delta 2=$______；
$(2)$若$\left(-3\right)\Delta \left(x-1\right)=\left(x-1\right)\Delta \left(-3\right)$,求$x$的值；
$(3)$试比较$\left(-3\right)\Delta x^{2}$与$x^{2}\Delta \left(-3\right)$的大小.