class MyModel(nn.Module):
    def __init__(self, model1, model2, model3):
        super().__init__()
        # You can also pass a list, rather than 
        # seperate models
        self.model1 = model1
        self.model2 = model2
        self.model3 = model3
    def forward(self, x):
        out1 = self.model1(x)
        out2 = self.model2(x)
        out3 = self.model3(x)
        return out1, out2, out3

This method involves moving all the models to GPU. So GPU memory would become bottleneck, but you will only have one set of inputs.
You can also do inference on CPU, which may be faster if your GPU RAM is small.