This is one of the best ways to make muscle gains, especially in the context of getting stronger.
For each exercise, take a given weight and try to sustain it through the given rep ranges, for the entire number of sets.
If you have a main lift, where you also have sets of 3 repetitions, try and bump those up to 5 repetitions.
Here’s an example:
Exercise: Bench Press
Sets: 4 working sets
Reps: 5,5,3,3
Set 1 – 5 reps, 80 kg
Set 2 – 5 reps, 80 kg
Set 3 – 3 reps, 80 kg
Set 4 – 3 reps, 80 kg
As you progress through this lift, try to complete 4 sets of 5 reps with 80 kg (or whatever your working weight is), by increasing the reps from 3 to 4 and 5, in time, on the last two sets.
Once that happens, bump up the weight with 2-5 kg, and aim to progress up to 4 sets of 5 with that weight.
When you do that, odds are that you will be able to get 5 reps on your first set, followed by sets of 3-4 reps on the last sets.
In time however, you will gain strength and you will be able to increase the repetitions.