You can combine PDFs on a Mac using a Python script that comes with Automator, /System/Library/Automator/Combine PDF Pages.action/Contents/Resources/join.py.

join.py -o out.pdf in1.pdf in2.pdf in3.pdf

Of course, the directory won't be in your path, so create an alias, or a link in /usr/bin, or use the full path, or some other method you prefer.