Read up on progression. You should be "progressing" in some way every workout, whether by increasing reps, weight, sets, intensity, or decreasing rest time. How you progress depends on your routine and current goals, and if you're not progressing, you're either not lifting hard enough, your workout is not proper, or some other part of your life is inadequate (diet, sleep, stress, etc.)
If you're doing just 3x8, I'll wager you're fairly new to lifting and would recommend progressing with both added weight and added reps intermittently. I.e. one workout you could do say squats with 3x8 for 135lbs. Next workout, do 3x8 for 140lbs. Next workout shoot for 140lbs for 3x9.
Or, better yet, focus for a month on getting your reps up from 8 to 12 per set. If you hit 12 for all three sets before the end of the month, up the weight so that you can only do 8 reps comfortably and start over. At the end of the month, switch up your routine and try 5x5 or 4x6 or some other rep scheme that your body is not used to and focus on strictly increasing weight each workout.