Do you mean you are cycling 5x1, 3x3 and 1x5 on your heavy day? Or is that what you are doing throughout the week? If it's the later, the 450x1x5 and 400x5x1 are both heavy workouts. See table 7-1 in PPST3, assuming 450 is close to 1RM (within maybe 5% since you're doing singles across) you're doing a set of 5 at roughly 85%, which still is a heavy set.
Furthermore, while the 360x3x3 is lighter and in the appropriate range, you're doing nearly twice the volume as the other 2 days. Light day should probably have NO MORE volume than the other days, and probably less. It may have been working OK for you because your heavy and medium days are pretty low volume at 5 reps each, but I would guess that you want more volume either on heavy day, or spread between heavy and medium days.
My understanding for tapering/peaking, is that you would reduce overall volume for a week or two (my contention is that for an intermediate, a single week is all that is needed) while keeping intensity high. Not really sure how that would fit into your current program, as volume is already low with pretty high intensity twice a week. I would suggest maybe trying some singles or doubles on medium day at planned weight, instead of a set of 5.