Sure, if you want. I have 5 AS levels though I went about it in a roundabout way (essentially 4*AS in Year 12, but 1*AS and 4*A2 in Year 13, so the workload of 5 subjects was in Year 13) and because I enjoyed the subjects it was never particularly arduous or stressful. In fact by working hard earlier in the year I was able to be staggeringly lazy in the Spring term, during school time just having fun with friends rather than revising for the most part. So the point is the time frame in which you have is far more than enough. But if you don't really like all the subjects, obviously it's pointless.