Don't forget you need to raise the sugar content of the mix. You need at least 10% ABV for the wine to keep well.
Plums are not a high sugar content fruit.
You have a Hydrometer correct? Once you get the plums mashed and water added to 3 gallons you need to check the SG. Add sugar and stir well. Also remember that if you have exactly 3 gallons volume when you start, you are going to be short on volume when you remove the mesh bag and the gross lees after fermentation completes. For a 3 gallon batch I would try to have the starting volume at about 3 1/2 gallons or maybe a little more. If you don't add any sugar until you've raised the volume to 3 gallons with the plums and the water, then the sugar, when dissolved will help raise that volume. For my batch of 3 gallons I had to add 14 cups of sugar to get the SG to 1.095. That will provide an ABV of about 13%. Perhaps a starting SG of 1.080 would be a good place to aim. That would be an ABV of 11% if it ferments totally dry.
Tonight I racked that plum wine off of some lees and I got to taste a little of the extra I had... perfect. Don't think it will need any sweetening at all.
This is the recipe I used. (I used a can of plum wine base plus 6 fresh plums for my batch of 3 gallons.
1 x 96 oz can Vintner's Harvest Plum wine base*
6 fresh plums destoned and mashed
2 tsp Acid Blend
1 1/2 tsp Pectic Enzyme
2 1/2 tsp Yeast Nutrient
3/4 tsp Wine Tannin
14 cups Sugar
Starting SG 1.095
Starting pH 3.40
*Before starting I did not treat the mix with K-Meta (Campden Tabs) for a fresh plum/wild plum batch I would prepare the mix above then treat with 3 Campden tablets crushed and dissolved in a couple of ozs of water. Let that treated must set overnight in your fermentation bucket with a cloth cover tied over the top.
The following day 24-36 hours later, prepare a yeast starter to rehydrate the yeast. I normally use about 1 oz of the juice 2 ozs very warm water and about 1/8 tsp of the yeast nutrient - mix well then add the yeast and let it sit for about 30 mins or longer. You should see some bubbling happen the yeast wakes up and rehydrates.
While the yeast starter is getting fired up I take one more set of measurements of the SG and the pH to make sure where I'm starting from. Then I pitch in the yeast stir and recover with the cloth and tie that down. No airlock is needed until the fermentation has finished.
Stir daily for at least the first 3-4 days and keep pushing down the mesh bag with the plum mash. That helps extract all the flavors and sugars from the plums.
Make sure the room temperature is in the right range for the yeast you are using.
Again good luck and keep us posted and peppered with any questions you have.