There are 25 choices for president. Then we have 25-1 = 24 choices for the secretary, and 24-1 = 23 choices for treasurer. This countdown (25,24,23) is because any given person cannot serve in more than one position.
Multiply out those values: 25*24*23 = 13800
Side note: you could use the nPr permutation formula as an alternative path. Plug in n = 25 and r = 3.
