解:
$ (1)$由题意,得$M(3)+M(4)=(-3)^3+(-3)^4$
$=-27+81=54$
$ (2)$由题意,得$3M(2024)+M(2025)$
$=3×(-3)^{2024}+(-3)^{2025}=3^{2025}-3^{2025}=0$
$ (3)3M(\mathrm {n})+M(n+1)=0,$理由如下:
由题意,得$3M(\mathrm {n})=3×(-3)^n=-(-3)×(-3)^n$
$=-(-3)^{n+1}$
$M(n+1)=(-3)^{n+1}$
所以$3M(\mathrm {n})+M(n+1)=0$