The First Number:
The first number is for the negative or eccentric phase - in other words, when you are lowering the weight or when you are moving in a direction opposite to the muscle contraction. For a squat and a bench press, this would mean lowering the weight. For a cable row, this would mean returning the plates to the stack.
The Second Number:
The second number is the pause after the first phase is complete - for example, in the bench press, a pause as the weight is held stationary just above the chest.
The Third Number:
The third number refers to the concentric or positive phase - the contraction. For a bench press, this would be driving the bar upwards. A number of 1 here typically means, "explode" - in other words, you may do it faster than 1 second.