The following works for me.
sudo apt-get install python3.5-venv
python3.5 -m venv --clear ./mypy3.5/
source ./mypy3.5/bin/activate
It uses the
(mypy3.5) $ which python
/home/ubuntu/mypy3.5/bin/python
But there is a gotcha which might have been your problem. The python3 -m venv uses soft links to how your python resolves in your environment. I had Python 3.3, 3.4 and 3.5 installed in /usr/local so the /usr/local/bin/python3 would change and break my Python3 venv. Note that "python3" is evaluated for the environment not for an absolute path. To be careful, when there are more than one Python 3 on you system, create your virtual environment with an explicit path like the following.
/usr/bin/python3.5 -m venv --clear ./mypy3.5/
source ./mypy3.5/bin/activate
ls -l $(which python3.5)
/home/ubuntu/mypy3.5/bin/python3.5 -> /usr/bin/python3.5*