You could always flag anything else in the system that you cannot handle yourself. Every helpful flag counts towards the badge.
For posts this will of course be
- Spam
- Offensive
- Not an Answer
- Needs ♦ moderator attention
And then there's the possibility to clean up comments. Go looking for too chatty or obsolete ones on older questions. Like people saying just
Thank you, what a great solution!
which would be a too chatty one.
Or this conversation would be obsolete:
There is an error in your answer in line...
Thanks, I fixed it!
Keep in mind though that if there are lots of chatty or obsolete comments on a question or answer you should flag the post for moderator attention and describe the situation instead of flagging each single comment.
Somewhere here on Meta (I cannot find it right now) I saw a statement by a moderator though, saying that you should flag the post only if all of the comments are flag-worthy. If there is one comment worthy to keep, just flag the other comments, but not the post. The reasoning behind that was that there's a moderator tool available that can remove all of the comments at once - which cannot be used if some comments should be deleted while other shall remain.
So you see: There's plenty of opportunity to get the Marshal-badge even after reaching 3k reputation.
marshal 3000
, didn't find anything relevant. Then I searched forflag 3000
and there it was on the top.