Put your multiple inputs into a tuple. (x1, x2, x3).
nn.Squential will work exactly the way you know, as it will look like you gave it one input.
In your custom layers simply unpack the tuple and if you need
multiple outputs return out1, out2, out3 as this will again become tuple.
If you want to be more pythonic about the unpacking you can surely go for creating your
custom subclass of nn.Sequential and use module(*tuple)