One week user of Ubuntu v15 and I have tried to install the file Python-3.5.0a1.tar gz but without success. The following points have arisen.
1). The file was downloaded. OK
2). The file was extracted to the Desktop as Python-3.5.0a1
3). Using sudu apt-get install Python-3.5.0a1, the message 'file not found'. Why not!
4). Looking into the extracted files it seems that this is a source package and therefore cannot be installed as is?
5). Finally, using the bash command 'find' it could not find the above file either.
Can someone kindly sort this out for me.
Thanks.

RTFM - or see https://www.python.org/dev/peps/pep-0478/
It seems unlikely that any distro is going to support alpha software releases.
This explains points 3 & 4, point 5 is your problem.

You will have to compile the downloaded source yourself if the latest available Python 3 package is too dated for you.