Some of my pictures are blurry using a cannon rebel 3. how can i fix this problem?

There are several possible causes for blurry pictures.
1) Camera motion. If everything is blurry, it's most likely because the camera moved while the picture was being taken. Make sure your shutter speed is fast enough if you're handholding the camera (longer lenses require faster shutter speed). Alternatively, put your camera on a stable surface, such as a tripod or a tabletop.
2) Subject motion. If the subject is blurry but the background is sharper, then the subject may have moved while the picture was being taken. Use a faster shutter speed. Raise the ISO if needed. There are limits, of course. If you're trying to photograph a fast-moving object in the dark, you probably won't be able to.
3) Improper focus. If the subject is blurry but something else in the picture is sharp, then the camera probably focused on the wrong object. Move the focus selector to the subject you want in focus. Alternatively, center the viewfinder on the subject, press the shutter button halfway to lock focus, then reframe and shoot.
Without any more specific information as to what and how your pictures are blurry, I'm afraid the above is all I can give you.