Many of the git objects in https://launchpad.net/cordova-ubuntu/3.0/apha1/+download/cordova-ubuntu.tar.gz have read-only (0444) permissions, which causes the failure output below. I suggest either removing the git metadata from the tarball or running "chmod --recursive o+w .git" before creating it.
$ cordova --verbose platform add ubuntu
Requesting {"url":"https://launchpad.net/cordova-ubuntu/3.0/apha1/+download/cordova-ubuntu.tar.gz","strictSSL":true}...
Downloading cordova library for ubuntu...
rm: could not remove file (code EACCES): /home/martini/.cordova/lib/tmp/tmp_cordova_13252_1422405364399/cordova-ubuntu/.git/objects/91/2062313a0e8d9fb64803544c7aed186c2501f7
rm: could not remove file (code EACCES): /home/martini/.cordova/lib/tmp/tmp_cordova_13252_1422405364399/cordova-ubuntu/.git/objects/91/2062313a0e8d9fb64803544c7aed186c2501f7
rm: could not remove directory (code EACCES): /home/martini/.cordova/lib/tmp/tmp_cordova_13252_1422405364399/cordova-ubuntu/.git/objects/91
rm: could not remove file (code EACCES): /home/martini/.cordova/lib/tmp/tmp_cordova_13252_1422405364399/cordova-ubuntu/.git/objects/91/2062313a0e8d9fb64803544c7aed186c2501f7
rm: could not remove directory (code EACCES): /home/martini/.cordova/lib/tmp/tmp_cordova_13252_1422405364399/cordova-ubuntu/.git/objects/91
rm: could not remove directory (code EACCES): /home/martini/.cordova/lib/tmp/tmp_cordova_13252_1422405364399/cordova-ubuntu/.git/objects
rm: could not remove file (code EACCES): /home/martini/.cordova/lib/tmp/tmp_cordova_13252_1422405364399/cordova-ubuntu/.git/objects/91/2062313a0e8d9fb64803544c7aed186c2501f7
rm: could not remove directory (code EACCES): /home/martini/.cordova/lib/tmp/tmp_cordova_13252_1422405364399/cordova-ubuntu/.git/objects/91
rm: could not remove directory (code EACCES): /home/martini/.cordova/lib/tmp/tmp_cordova_13252_1422405364399/cordova-ubuntu/.git/objects
rm: could not remove directory (code EACCES): /home/martini/.cordova/lib/tmp/tmp_cordova_13252_1422405364399/cordova-ubuntu/.git
rm: could not remove file (code EACCES): /home/martini/.cordova/lib/tmp/tmp_cordova_13252_1422405364399/cordova-ubuntu/.git/objects/91/2062313a0e8d9fb64803544c7aed186c2501f7
rm: could not remove directory (code EACCES): /home/martini/.cordova/lib/tmp/tmp_cordova_13252_1422405364399/cordova-ubuntu/.git/objects/91
rm: could not remove directory (code EACCES): /home/martini/.cordova/lib/tmp/tmp_cordova_13252_1422405364399/cordova-ubuntu/.git/objects
rm: could not remove directory (code EACCES): /home/martini/.cordova/lib/tmp/tmp_cordova_13252_1422405364399/cordova-ubuntu/.git
rm: could not remove directory (code ENOTEMPTY): /home/martini/.cordova/lib/tmp/tmp_cordova_13252_1422405364399/cordova-ubuntu
rm: could not remove file (code EACCES): /home/martini/.cordova/lib/tmp/tmp_cordova_13252_1422405364399/cordova-ubuntu/.git/objects/91/2062313a0e8d9fb64803544c7aed186c2501f7
rm: could not remove directory (code EACCES): /home/martini/.cordova/lib/tmp/tmp_cordova_13252_1422405364399/cordova-ubuntu/.git/objects/91
rm: could not remove directory (code EACCES): /home/martini/.cordova/lib/tmp/tmp_cordova_13252_1422405364399/cordova-ubuntu/.git/objects
rm: could not remove directory (code EACCES): /home/martini/.cordova/lib/tmp/tmp_cordova_13252_1422405364399/cordova-ubuntu/.git
rm: could not remove directory (code ENOTEMPTY): /home/martini/.cordova/lib/tmp/tmp_cordova_13252_1422405364399/cordova-ubuntu
rm: could not remove directory (code ENOTEMPTY): /home/martini/.cordova/lib/tmp/tmp_cordova_13252_1422405364399
CordovaError: Unable to fetch platform ubuntu: Error: EACCES, mkdir '/home/martini/.cordova/lib/tmp/tmp_cordova_13252_1422405364399/cordova-ubuntu/.git/objects/ec'
at /usr/share/cordova-cli/node_modules/cordova-lib/src/cordova/platform.js:121:27
at _rejected (/usr/share/cordova-cli/node_modules/q/q.js:797:24)
at /usr/share/cordova-cli/node_modules/q/q.js:823:30
at Promise.when (/usr/share/cordova-cli/node_modules/q/q.js:1035:31)
at Promise.promise.promiseDispatch (/usr/share/cordova-cli/node_modules/q/q.js:741:41)
at /usr/share/cordova-cli/node_modules/q/q.js:557:44
at flush (/usr/share/cordova-cli/node_modules/q/q.js:108:17)
at process._tickCallback (node.js:415:13)
