If I do get you right, do as followed:
Define a flight phase for your elevator function and adjust a delay of 3 seconds in the menu.
Define a mixer from elevator to elevator and set it to (S) Selected.
If the elevator down flight phase is active, select a constant curve and a master value.
With adjustment of these two parameter you can put the elevator down to the required position.
If the flight phase is activated, the base position will change to the new defined position by time, meaning the servo will go down to the new base position step by step over the 3 seconds.
I hope I was able to help you so far and got you right.
Regards Bodo