Put one cup of the juice in a saucepan, and sprinkle the agar over it; allow it to soften for a few minutes. Heat on medium-high, stirring, until the agar is completely dissolved and the juice begins to boil.
Stir in the remaining juice and the agave (*optional) and remove from heat. Allow the agar mixture to cool slightly while you prepare the fruit. Be sure to stir the juice every minute or so.
Remove the stems from the strawberries and slice them lengthwise. Place them on the bottom of a loaf pan, prettiest sides down. Slice the bananas and place them on top of the strawberries. Put the blueberries in an even layer on top of the bananas.
Gently pour the juice mixture evenly over the berries. Working quickly, distribute the mixture evenly in pan.
Cover with plastic wrap and refrigerate until completely chilled and jelled.
Carefully run a knife along the edge of the mold (you may want to lower the pan into a large bowl or sink full of hot tap water for a few seconds to free the mold)
Invert over a platter