Just use the construct you added. You only need to inherit from nn.Module, and call the __init__ from it
class MyComplexModel(nn.Module):
    def __init__(self, net1, net2, net3):
        super(MyComplexModel, self).__init__()
        self.net1 = net1
        self.net2 = net2
        self.net3 = net3

    def forward(self, input):
        return self.net3(self.net2(self.net1(x) + 1)**2) / x