How much is a 1980 Remington model 1100 3 with 30 plain barrel worth?

It depends on condition, rarity, and guage. If we are talking like you just dug it out of a time capsule and took it out of the original box. Mabye in the $500 dollar area. That is a high gloss engraved receiver and bolt, high gloss stock with no splits or dents. If it is in 80% condition around $250. It will vary depending on who is trying to get it from you also.. I would keep it. I have one myself that my Grandfather left me. Every time I take it out people drool. Even those who have thousand dollar Italian guns. The gun is as old as I am, but still runs with todays "best"