Need more info. but

If you are low by 3" at 100 yds. and your scope is 1/4" increments per click/graduation then to hit the center from which you measured your 3" you would need to go up 12 clicks. Adjust your scope to where you want the bullets to impact.

If you are 3" low at 25 yds. thats a whole nother deal. Remember that the bore of your rifle is actually below you line of sight by 1.5 inches or so...sometimes more.

Also if your bullets are hitting left, adjust right.