Many times when Group 2 is called (only EXP's) then group 3 is immediately called, without enough time for EXPs to board without being rushed by Group 3. So in practice Group 2 really boards with Group 3.
On flights with an F cabin, EXPs are already Group 3. Group 1 for F, Group 2 for J, Group 3 for EXP, then Group 4 for PP, Plat.