Put one cup of the juice in a saucepan. Sprinkle the agar over it, and allow it to soften for a few minutes. Then stir, heat on medium-high, 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. It will start to jell immediately, so work from side to side to make sure it's distributed evenly. Cover with plastic wrap and refrigerate until completely chilled and jelled. Invert over a platter to serve (you may need to lower the pan into a large bowl of hot water and run a knife along the inside of the mold to loosen the edges.)