user36:

and I get only one prediction for a batch size of 100 (instead of 100X3)


I guess you are reshaping the activation in the forward method of your model and might change the batch size to 1. If you are using x = x.view(-1, SHAPE) to flatten the tensor, then replace it with x = x.view(x.size(0), -1) which will keep the batch size and might raise a valid shape mismatch error in another layer.