Keras training only specific outputs
You have to create 2 different models like this
model1 = Model(input=input, output=[out1,out2])
model2 = Model(input=input, output=[out1,out2,out3])
You compile both but only fit the first. They will share the layers so model2, even if it wasn't trained, will have the weights learned from model1. But if there is a layer in out3 which is trainable but not in the flow between input and out1 and out2 of the graph, that layer wont be trained so will stay wirh its inital values.
Does that help? :-)