My husband and I can eat a whole bulb of garlic at one meal (roasted in oil) but we probably shouldn't.
Why don't you just ask the person you are trading with what you are thinking and see what she says. If she wants to do one for one would you say no? If you mention your reasoning for 2 or 3 to 1 see what she says. Maybe she has no issues and will give you 3 without flinching or maybe you have to barter Isn't that what it is all about?