It's not clear from your message, but are these JPGs straight out of the camera or processed from RAW? If from the camera, do you have the original, full resolution JPGs safely stored away? As others have already noted, the concern is that you may be destroying data in your source image files that cannot be restored.
Obviously you can downsize images. But if you trying to re-save your JPGs at file size 3 back up to 8, this won't restore detail, the data is gone forever.
It sounds like hard drive storage space is lacking. Additional internal or external drives are relatively inexpensive. These would be good insurance in any case so you can back up duplicates of your pictures for safe keeping. CDs or DVDs are other options.
If you prefer to shoot JPGs in camera and you don't need the full resolution images, shoot at a large or medium resolution instead. This also keeps you from having to later "dumb down" images in Photoshop, which destroys image data.
If you are processing RAW to make JPGs, most converters allow you to make JPGs at less than full resolution, or at a quality lower than 100%.
Paul