what type of fermenter do you have, bucket or dj's? What method using to prepare rhubarb, boiling water and sugar, steep 3 days stiring and strain. Boiling water on rhubarb, simmer and strain, disolve sugar then add to fermenter or no strain add lot to fermenter and strain after 7 days ferment.
You have 11lbs just short for requirement for 3 gallons, but if in bucket fermenter then 3 gallons would propbably be ok, some prefer to use more rhubarb so 2 gallons if you want a stronger flavour.
Either 1.7 kg or 1.5 kg per gallon sugar depending on the method you choose and what yeast you use high abv' 18-21% tolerant yeast may well use up 1.5kg sugar making it dry, 1.7kg will give you extra sweetness. I prefer it between an specific gravity of 1.020 - 1.010, some prefer 1.030.
Basically it's down to you and the quality of rhubarb and the yeast you plan on using.