First of all welcome to the Pytorch Community Rui_Li 
The first dimension of Pytorch Convolution should always be the the number of channels (3) for the input image , While on the other and the first dimension of the inputed image should be the batch_size(36). So do not let this confuse you . Furthermore the convolution operation will be done 36 number of times for all of the 36(batch size) images in parallel.