Apparently pupils don't adjust terribly quickly:

"Briefly, it was found that when light is flashed on a normal eye that is accommodated for the dark, there occurs a latent period of 0.1875 second before the pupil begins to contract. Then there follows a rapid primary contraction for 0.4365 second at the rate of 5.48 mm. per second. This is succeeded by a secondary contraction of 0.3125 second at the slower rate of 1.34 mm. per second"

https://jamanetwork.com/journals/jama/fullarticle/286369

Doesn't explain 3 seconds, but perhaps 1 second or so.