My records show Tell 3 spring dimensions to be: flat wire .195 X .048", OD .80", approx 20 coils total, free length just over 4", cocked length 1.3".
As a replacement on a Tell whose spring was broken I once used a portion of a Britannia spring (flat wire .150 X .075, OD .80, cut to 14 coils total, free length 3.44. My choice of length was aimed at replicating the solid height of the original ( ~ 1.07) and a note says this gave "quite powerful" performance.
An old Quackenbush spring I have is about .170 X .047, OD .72 which does not fit over the Tell 3 piston rod of .392 dia - maybe a long-reach boring tool could reduce the rod diameter to use such a spring?
Hope this helps,
Don R.