I can comment on the Need for 3 ELINT Aircraft
In my Experiance Roughtly
25% of any Fleet are in Mainternance
25% Training for aircrew
and the rest on Ops
From that it is easy to work out 3 aircraft is a minimum for any fleet
1 in Mainternance (Equlized or Major with regards to the Nimrod, not including Non scheduled Mainternance)
1 for Aircrew Training, Ops is required
1 on Ops