Locking a shared Exel document for editing by one person at a time

We share an excel document on Dropbox, Three people access and update it. If two people open at the same time it saves two versions on Drop box. how do I lock it for editing by one person at a time only whilst letting the other still view it if they need to? On the exel tab under review it is not selected as allow people to edit it at the same time so they shouldn't be able to. How do I fix this?