Focus on the subject you dont understand the most. For me it was physics, so I worked most on physics, then on chem then bio, as bio came easily to me.
As a rule of thumb, start revising early, go over the basics of everything but not the detail first. When you have a good understanding of all the basic concepts of everything, then go over everything and add a bit of detail. Then do the same again.
That way, you build your knowledge for all the topics at the same time, and you'll probably notice a bit of overlap between the modules of each subject.
The other benefit of doing this method is that you won't forget a whole module when you've finished revising it and started revising another. Think of it as layering a sandwich. The bread is the first basic concepts, then the butter is the next bit of detail, then the ham is the next bit.