why 3 allrounders?
Well, all those 3 allrounders are spinners who can get more assistance at the seddon park. Seddon park track has a small covering of grass which will be no more post Day 1 after which the fast bowlers will have to work hard for wickets. And, jp can be more handy with the bat as well.
PS: Rain is in forecast for all the 5 days of the test match