1 pound of honey will give you roughly 0.035 Gravity points when added to water (SG 1.000) for a total volume of 1 gallon. In essence you must remove 1.5 cups of water from the gallon as the honey will add that much volume back. (A cup of honey weighs roughly 12 oz)
So...
3 pounds of honey dissolved in water for a total volume of 1 gallon looks like this... 0.035 X 3 = 0.105 + 1.000 = 1.105
NOTE - These are rough estimates as honey is about 80% fermentable sugars. Some honey is a little more or less depending upon the water content, solids and misc other stuff. As well the constant factor of 0.035 is not exactly linear at higher levels. But for me is a reasonable estimate.
There are a bunch of calculators out there on the Web that might take all that into consideration.
This site explains honey and SG along with weight of honey pretty well and in terms I could understand.
https://www.meadmakr.com/just-how-much-honey-is-in-mead/