The first two are simple to install (if you happen to be running an x86_64 machine with a ton of RAM) - follow the procedure at WebAssembly.org, allowing a couple of hours). If you're operating under constrained resources, take a look at TIL - emscripten. For ARM systems, definitely read that document - until emsdk correctly identifies architecture rather than just bitness, you're on your own in terms of node.

The third, OpenCV, needs to be separately compiled to llvm archives and moved
somewhere in the path emscripten uses for linking. The simplest way to do this
is to replicate the logic at docker-alpine-emscripten/opencv.
Essentially, run the wrapper.sh script (emcmake with a lot of obtuse, poorly understood parameters, followed by emmake) in the root directory of a copy of the opencv source, then copy the include headers for both opencv and opencv2 to their expected locations in the emscripten tree (i.e. /system/include/(opencv|opencv2)).