Is it possible to connect three neural networks in Matlab?
If you want to combine the results from three different Neural Networks to "boost" the performance :) , you might want to look at the different Ensemble Learning Methods as I mentioned earlier. 

Which method you should use, depends on how you share or divide the training data between the three NNs. For example if the NNs are trained on same data but have different parameters, you can look at simple voting ( if you are doing a classification task) or averaging ( if you are using them for regression).

The more advanced methods like AdaBoost divide the training data between the classifiers. You can read about it in Boosting Neural Networks