Ideally one would differentiate h and find the instantaneous velocity. Apparently "slope of secants" is the approximate idea of differentiation. Sois, "I found the instantaneous velocity by creating a table and using numbers that get closer and closer to 3 seconds until I can reach a conclusion" is called "slope of secants" method.my guess

Lets call the height h(t), since its a function of time, then instantaneous velocity at t=3 is , where x can be any small number(like 0.01,0.001 etc)