Spray the inside of a slow cooker with cooking spray. Spread onion slices in the bottom of the cooker.

Pat roast dry and sprinkle all over with salt and pepper. Spread flour onto a work surface and roll roast in flour to coat all sides, using your hands or a small sturdy plate to pound flour into the meat. Place floured roast into the slow cooker on top of onions.

Whisk together beef gravy mix, ranch dressing mix, and Italian dressing mix in a bowl, then add water and whisk until smooth. Pour over roast. Distribute carrots around the meat.

Cover the slow cooker, set to Low, and cook until roast is tender and gravy has thickened, about 8 hours.