You want to scale down 8 x 11" to 3 x 5" which would downscale the width to 0.375 of the original width and the height to 0.4545 of the original height. That's going to make all of the new images look funny looking because the proportions of the existing images would not be preserved. Downscaling an 8 x 11" image to 4 x 5.5" would preserve the original proportions of the image and reduce its width and height to 50% of its original width and height.
There is a one-line command in ImageMagick (sudo apt install imagemagick) which can batch resize multiple images. Change directories using cd to the directory that contains all the images to be resized and run a command that has the following form.
mogrify -resize widthxheight! *.jpg
If your images are .png images, then replace .jpg with .png in the above command.
Click an image and select Properties -> Image tab as shown in the screenshot.
The image in the screenshot is 500x500 pixels. Applying the downscaling formula would reduce the width to 500 x 0.375 = 187.5 pixels and the would reduce the height to 500 x 0.454545 = 227.25 pixels. image-01.jpg is a square image, but it will be a rectangular image after it is resized.
Substituting 187.5x 227.25 for width and height produces the following command:
mogrify -resize 187.5x227.25! *.jpg
The mogrify command cannot resize to fractions of a pixel so the resulting files will have dimensions of 188 x 227 pixels.
This mogrify command resizes every .jpg file in the current directory and it replaces every existing image by a new resized image. To avoid losing all your original files you must make backups of all the original images before you resize them! I recommend storing all your original images in a safe place like on a USB flash drive, so that you don't accidentally resize them.
