Two interesting links for reviews that include a section measuring the surface temperatures of the retina MacBooks (first is 2015 and second is 2016 version, both are base models). They give the rooms ambient temp underneath for each set of measurements.
http://www.notebookcheck.net/Apple-MacBook-12-Early-2015-1-1-GHz-Review.143178.0.html
http://www.notebookcheck.net/Apple-MacBook-12-Early-2016-1-1-GHz-Review.164797.0.html
So it looks like the core m3 macbook runs slightly cooler under load than the older core M. Note that the first review was with OS X Yosemite and not El Capitan that could improve performance of the older version.
http://www.notebookcheck.net/Apple-MacBook-12-Early-2015-1-1-GHz-Review.143178.0.html
http://www.notebookcheck.net/Apple-MacBook-12-Early-2016-1-1-GHz-Review.164797.0.html
So it looks like the core m3 macbook runs slightly cooler under load than the older core M. Note that the first review was with OS X Yosemite and not El Capitan that could improve performance of the older version.