jespersh738130d96MP at rgba becomes 384MB. If you're on the 16b per component, then 768MB minimum. To get to 3GB then it would need to work with 4copies of the same image. Possible? Maybe, I am not super familiar with the internals.
fuckwit135530d@Conrad Imagemagick needs to allocate a few things and pulls everything into memory in order to be faster.
Meaning the full size of your output image has to be allocated. The images you are stitching together and each of them twice as you specified the -clone 0 flag.
I'm currently not sure if your resize operation will also allocate another buffer though.
This will quickly add up.