You can install opencv from source.
Follow this link to do so.
Or you may need to upgrade your pip3 using the following command
pip3 install --upgrade pip
EDIT
For completeness(and in case the link is broken) I've listed here steps to compile and install OpenCV from source on Ubuntu(Tested on Ubuntu 14.04 LTS with python 3).
Step 1 Update the packages
sudo apt-get update
sudo apt-get upgrade
Step 2 Install dependencies
sudo apt-get install build-essential cmake git pkg-config # Developer tools required to compile opencv
sudo apt-get install libjpeg8-dev libtiff4-dev libjasper-dev libpng12-dev # Libraries required to read various image format from disk
sudo apt-get install libavcodec-dev libavformat-dev libswscale-dev libv4l-dev # Libraries required to read various video formats
sudo apt-get install libgtk2.0-dev # Required by opencv for GUI features
sudo apt-get install libatlas-base-dev gfortran # Packages used by opencv to optimize various functions.
pip3 install --upgrade pip
Step 3 setup virtual environment(using conda)
conda create -n opencv-exmaple-env python=3.6
source activate opencv-exmaple-env # Activate the envirnoment
Step 4 Install packages required to compile opencv
sudo apt-get install python3.6-dev # If the python version is not 3.6 then make changes to this command accordingly.
pip install numpy # This should be done after the environment in Step 3 is activated
Step 5: Build and install OpenCV 3.0 with Python 3.4+ bindings
5.1 Clone the opencv source
cd ~
mkdir opencv-source
cd opencv-source
git clone https://github.com/Itseez/opencv.git
cd opencv
git checkout 3.3.0 # Branch you want to compile from
5.2 Clone Opencv Contrib rep
Contains exptra functionalities such as standard keypoint detectors and local invariant descriptors (such as SIFT, SURF, etc.)
cd ~
mkdir opencv-contrib
cd opencv-contrib
git clone https://github.com/Itseez/opencv_contrib.git
cd opencv_contrib
git checkout 3.3.0 # The version you want to compile
5.3 Compile,build and install
cd ~/opencv-source/opencv
mkdir build
cd build
cmake -D CMAKE_BUILD_TYPE=RELEASE \
-D CMAKE_INSTALL_PREFIX=/usr/local \
-D INSTALL_C_EXAMPLES=ON \
-D INSTALL_PYTHON_EXAMPLES=ON \
-D OPENCV_EXTRA_MODULES_PATH=~/opencv-contrib/opencv_contrib/modules \
-D BUILD_EXAMPLES=ON ..
make -j4
sudo make install
sudo ldconfig
5.4 Link installed opencv object file to python site packages
ln -s /usr/local/lib/python3.6/site-packages/cv2.so /path-to-python-sitepackages-of-the-environment/cv2.so
6 Verify installation
import cv2
If the above code runs without error then opencv is installed successfully.