First, I suggest to let the rejected editor know via comment that you rejected their edit by mistake. You can let them know that, if they suggest the edit again, you will approve it.
See the FAQ:
Additionally, moderators can override the outcome to any suggested edit, in addition to the post author. They are the only ones who can override outcomes on suggested edits to tag wikis, as those have no specific owner.
Asking in chat is not always possible / successful, but you can simply flag your own post for moderator attention and explain that you rejected the edit by accident.
To the question whether reviews could be undone, Shog9's answer was:
Note that "undo" is tricky in all cases, even the relatively benign "do nothing" ones: if the review has completed, and particularly if significant time has passed since it completed, changing your response may not be possible.
I think the cost of this outweighs the advantages at this point.
In your case, the following happened:
- Suggested edit was approved by 2 reviewers -> creating revision 2
- When notified of the edit, you clicked "reject" and thereby overrode it -> creating revision 3
So you actually rolled back the edit, while it just looks as if you reviewed it.