I mean it depends where you're at with the content - if you're comfy with most of it just bash out as many past papers as you can (and mark them - this is the most important bit!).
If you have some content that you're not so sure with, then try and focus in on the bits that are achievable for you - you know what you're capable of. You want to try and brush up on everything you can, and leave off the stuff you'd need to relearn from scratch (but ideally look at it briefly - you might realise its not that bad).
As for cramming each one into x days, its better to try and get them spread out, so work on all three over the weekend and then dig into just bio on Monday so you're ready, then do the same with chem, if that makes sense. Interleaving like that can really help break up the day and make it easier to deal with, i find.