Hello,
expert in OpenCV programming here. It is my understanding that on AWS Lambda you don't actually have direct access to the underlying architecture, so you can't actually compile and install custom code (like OpenCV 3).
For what you need, I think an EC2 instance (or any other VPS provider where you can actually modify what is installed on server) is more appropriate.
Thanks and let me know if you are interested to work with me,
Paul