It’s less about reshaping but converting your input tuple (tensor1, tensor2, tensor3) to a tensor. What you can do is, for example:
input = torch.zeros(3, batch_size, encoder_size)
tensor[0], tensor[1], tensor[2] = tensor1, tensor2, tensor3

This assumes that tensor1.shape = (batch_size, encoder_size); same of tensor2 and tensor3, of course. If the shapes are (seq_len, encoder_size) then you have to adopt the code a bit. But I cannot tell which shapes your tensors have.