To add these three volumes, you simply add them together. Here are the solution steps:
Let's start by adding the first two volumes:
2.125 ml + 5.84 ml = 7.965 ml
Now let's add the third volume to the sum of the first two:
7.965 ml + 1.428 ml = 9.393 ml
So, the sum of the volumes 2.125 ml, 5.84 ml and 1.428 ml is equal to 9.393 milliliters.