Hosted here: http://peaceful-ocean-71245.herokuapp.com/
The first task is stored as python/highest_product_of_3.py.
Assumption made: All arrays passed are valid (len >= 3, numbers only).
Note: Implementing this in linear time is fairly straightforward, but I figured doing it with a sort would be more legible.
The rest of the repo is the second task. The backend code was written in scala (using akka-http and circe), and is stored under src/main.
The backend just uses a stock parser combinator for parsing and evaluating the expressions, so I also wrote a Shunting-Yard parser in python (python/infix_to_postfix.py).
The frontend code was written in react (using create-react-app) and is stored under frontend.