If you are going to switch from bodybuilding style split to a powerlifting routine, I would do more than 3 week intervals. My guess would be you would like to get stronger, so when you return to your old routine, you will be able to use more weight, even at higher reps. I would do your powerlifting phase for at least 6 weeks, maybe 8, to try to make some significant strength gains.
If you were going to do peroid training with # of reps..say 6 and under
7 to 9
10 to 12 or higher then I would say 3 weeks would be a
good period for each. But I don't think 3 weeks of powerlifting every 10 to eleven weeks is long enough to make much gains in strength. If you are doing this just to change it up and keep from getting stale, them I guess your proposed periodization is ok.