These (mostly) run fine with their original PKGBUILDs from the Arch Linux repos, just adding aarch64 to the arch array - however they're a bit weird in general, which took some effort to fix:
PyPy3 makedepends on PyPy, and PyPy makedepends on itself, which is obviously a problem.
However, with minor changes to the PKGBUILD, pypy can compile with regular old python2.7, which can then be used to compile pypy3. Once you compile pypy with python2.7 you can even recompile it with the unmodified PKGBUILD since you now have pypy installed, but I'm unsure if this provides any benefits. The changes required to compile with python2.7 for pypy are as follows:
Line 25: replace
$this->bbcode_second_pass_code('', 'pypy ../../rpython/bin/rpython -Ojit --shared targetpypystandalone'')
with
$this->bbcode_second_pass_code('', 'python2.7 ../../rpython/bin/rpython -Ojit --shared targetpypystandalone')
Line 35: replace
$this->bbcode_second_pass_code('', 'pypy pypy/tool/release/package.py --archive-name pypy --targetdir .')
with
$this->bbcode_second_pass_code('', 'python2.7 pypy/tool/release/package.py --archive-name pypy --targetdir .')
and swap pypy in the makedepends for python27. That's all that's needed to successfully build and install pypy, which is then used to build pypy3 without modifications.