In a large skillet, combine strawberries and sugar. Cook over medium heat for about 5 minutes or till sugar dissolves. Add blackberries and blueberries. Cook about 5 minutes or till they just release their juices, but still hold their shape. (The mixture will look like a thick soup.)
Remove from the heat and let stand till cool. Stir in lemon juice and salt.
Line six 1/2-cup baking dishes or 6-ounce custard cups with plastic wrap. Cut a circle from each bread slice to fit into dish (18 circles in all).
Spoon 2 tablespoons of the berry mixture into each dish. Place a bread layer atop berry mixture. Repeat layers 2 more times. Spoon 2 tablespoons additional berry mixture atop the last layer of bread.
Arrange baking dishes in a shallow baking pan and cover with plastic wrap. Place a board large enough to cover them all on top. Weight board with something heavy. Chill overnight.