To get stronger, focus on one exercise and add resistance on that exercise. As you advance, apply “periodization” to your loading scheme.
Periodization has been well studied, but less attention has been given to exercise variety. Evidence shows that varying exercises may produce superior results.
One group performed one exercise (for example, squat) for 12 weeks using its 8 RM while increasing the number of sets from 4 to 6 to 9 for Weeks 1–4, 5–8, and 9–12, respectively. The second group used the same load and set protocols but did squats, deadlifts, and lunges. The group who did three different exercises gained significantly more strength even though total number of sets, reps, and weight loads were the same.
Three sets each of three different exercises will yield better strength gains than 9 sets of one exercise.
Exercise variety plays an important role in maximum strength development, particularly for larger muscle groups.