Node.js is the most popular of the three among modern tech stacks, and for good reason: it's simple, lightweight, and fast. Plus, being popular has its advantages: better documentation, libraries, and support when you get stuck (which you undoubtedly will as a beginner).
Pair it with ExpressJS for the best experience.
Rule out PHP since you aren't familiar with it, and learning it would take much time since PHP has turned out to be complex lately.
Node.js vs. Flask
Node.js is your go-to solution for developing speed-intensive websites and applications (eCommerce stores, chatbots, etc.). Flask isn't the ideal solution. Django (another Python-based web framework) is better than Flask.