How to calculate terminal velocity knowing three velocities at different points in time, with the time difference in between measured?

Help me r/physics, youre my only hope.

As the title would indicate, I have to find a formula for calculating terminal velocity when I know the instantaneous velocity of a falling ball at three points in time, with the time differences in velocities. The idea is that I should be able to find the change in acceleration with these values, which I have tried with this formula, and I will then use this to find when acceleration will be zero with this formula, thus the point in time when terminal velocity has been reached. However, I can just not get this to make sense. Are they correct? Should change in acceleration, and thus also the output terminal velocity, be negative? Please help me, I am desperate as this is determining for how I can analyse my essay. The predicted terminal velocity with the standard drag equation formula is ~20ms^-1.

If there is a better suited sub for this, please also guide me there as I need this info as fast as possible, but for any one of you this should really just be looking over and confirming, right?