import re
arr = ['Käuferprovision: 3 % zzgl. gesetzl. MwSt.', 'Käuferprovision: Die Courtage i.H.v. % 3,57 inkl. MwSt. ist']
rx = re.compile(r'\d+(?:[.,]\d+)*(?=\s*%)|(?<=%)\s*\d+(?:[.,]\d+)*')
for s in arr:
for m in rx.finditer(s): print (float(m.group().replace(',', '.')))
aW1wb3J0IHJlCmFyciA9IFsnS8OkdWZlcnByb3Zpc2lvbjogMyAlIHp6Z2wuIGdlc2V0emwuIE13U3QuJywgJ0vDpHVmZXJwcm92aXNpb246IERpZSBDb3VydGFnZSBpLkgudi4gJSAzLDU3IGlua2wuIE13U3QuIGlzdCddCnJ4ID0gcmUuY29tcGlsZShyJ1xkKyg/OlsuLF1cZCspKig/PVxzKiUpfCg/PD0lKVxzKlxkKyg/OlsuLF1cZCspKicpCmZvciBzIGluIGFycjoKCWZvciBtIGluIHJ4LmZpbmRpdGVyKHMpOiBwcmludCAoZmxvYXQobS5ncm91cCgpLnJlcGxhY2UoJywnLCAnLicpKSkKCQ==