I assume the 3rd second has a much greater drop than the first second....
Yes. Sorry, forgot to divide by 2! So half that for the first second
1/2*a*t^2 -= 1/2*9.8*t^2
So at 1 second, 4.8m
at 2 seconds, 19.6 m
At 3 seconds, 44.1m
So at 1000 fps, a horizontal shot will drop 44.1 m or 144.685 ft