Understanding multi-variable big O (time complexity)
You can remove the $-1$ from $(n-1)$, since $(n-1)dm=ndm-dm\leq ndm$. On the other hand, you can not simplify beyond $O(ndm+mv+v^2)$ unless you know more about the relations between the variables. $mv$ and $v^2$ are incomparable, depending on the ratio between $m$ and $v$ one may grow faster than the other. Similarly, your last step (removing $ndm$ in favor of $v^2$) is also incorrect.