Thanks for your reply, @ptrblck. As you can see i have three models to concatenate to the fc layer. So at forward propagation i add another x3,  is that correct ? Can torch.cat concatenate more than 2 models? Also in
self.classifier = nn.Linear(2048+512, nb_classes)

How you determined the 2048+512  ? If i add my third model what will be the number for this classifier?
Also I have defined the activaiton function inside my model, is it necessary to add the F.relu() again at the end?