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Putting a tank on your 3pt would be limited by the counter weight added to the front end. Your manual should indicate the lift capacity of the 3pt, so depends on what that is in gallons figuring 8lbs to the gallon for approximate weight. Add some for the sprayer frame.
Give out some info on what you have now, and what acreage you want to spray, and if you plan on a boom sprayer, or orchard-type sprayer. All could help with some answers for you.