I'm working in Amazon's Cloud9.
ec2-user:~/environment/flask_init $ python -V
Python 2.7.14
ec2-user:~/environment/flask_init $ virtualenv -p python3 venv
Running virtualenv with interpreter /usr/bin/python3
Using base prefix '/usr'
New python executable in /home/ec2-user/environment/flask_init/venv/bin/python3
Also creating executable in /home/ec2-user/environment/flask_init/venv/bin/python
Installing setuptools, pip, wheel...done.
ec2-user:~/environment/flask_init $ source venv/bin/activate
(venv) ec2-user:~/environment/flask_init $ python -V
Python 2.7.14
Why is the virtual environment not using Python 3?
Please note that this question is not a duplicate of this one. The issue was specifically to do with the way the Cloud 9 environment sets up Python alias.
I tried your flow on my machine and everything works as expected.
dluzak@Karol-PC:/tmp$ python -V
Python 2.7.12
dluzak@Karol-PC:/tmp$ virtualenv -p python3 venv
Already using interpreter /usr/bin/python3
Using base prefix '/usr'
New python executable in /tmp/venv/bin/python3
Also creating executable in /tmp/venv/bin/python
Installing setuptools, pkg_resources, pip, wheel...done.
dluzak@Karol-PC:/tmp$ source venv/bin/activate
(venv) dluzak@Karol-PC:/tmp$ python -V
Python 3.5.2
(venv) dluzak@Karol-PC:/tmp$
Nonetheless I personally use virtualenv as module when creating venv with python 3: python3 -m virtualenv venv. Maybe this would work.
You provided very little details. Have you installed virtualenv for both Python 2 and 3? Are you sure Python 3 interpreter works fine?
Edit:
After investigation in comments we found out that the problem was in bash settings configured by Amazon. It seams that Amazon configures bash (probably in ~/.bashrc) to replace python calls with an alias. To fix this a call unalias python before enabling venv is needed. It is described in Amazon docs