Say you already have a level 3 red grape. Can you harvest a field that has a 2-red-vine, and a 1-red-vine, and create a 1-grape and a 2-grape from it, as one harvest action?
No you could not create two red tokens from one harvest action. From the rules here you create up to one white and one red tokens on the crush pad.
To harvest a field, add the total value of all red grape icons on the vine to create 1 red grape token and then add the total value of all white grape icons on the vine to create 1 white grape token on your crush pad.
The rules are specific it is "1" and not 2 in smaller combinations.
In your situation you must try to create a value 3 red grape token and then devalue it.
If the grape icon already has a token on it when you harvest a grape at that value, you must devalue the grape token to the next available lowest icon.
In your example you would place a red token on the number 2 space.