Make wine from Blackberry juice only??

Someone is giving me 3 gallons of blackberry juice they squeezed. I want to make a 5 gallon batch. I'm looking for help on how much juice to use and what additives I need to use.

Thanks!

Not going to get 5 gallons of good wine out of 3 gallons of juice, unless you can get more juice from another source.

BlackBerry is one of my favorites.

With blackberries I was taught you want straight juice to get the right flavor. You could always do up a 3 gal batch and I think you would be happier with it than a watered down one.Here's a link to my 1st blackberry wine. I had a lot of great help through it all and was really happy with the outcome. I still have a few in the cellar.
Remember to take the basic steps and then add to it to make it your own if you want.
http://www.winemakingtalk.com/forum/showthread.php?t=38273

There is also a thing with the acid levels in blackberry. Did your source press the berries or steam them. Sometimes the acid levels are so high, a dose of potassium carbonate can help drop the pH down some. You could make an ok blackberry 3 gal of pure juice in 5 but 100% juice is more impressive though harder to balance, this is one reason why you almost always see blackberry wine as being made very very sweet. It also takes oak very well, and if you switched to making a melomel you could add 5 quarts of honey at the beginning and bring to vol up to 5 gal and make a very good melomel. WVMJ