I haven't tried Fable 3 yet, but I can speak to Fable 2 (F#) vs ReScript/ReasonML (OCaml).
F# was heavily inspired by OCaml, it was originally intended as "What would OCaml look like running on .NET?" so the languages themselves share a lot of similarities.
ReasonML pros:
- ReasonML compilation times were much faster than Fable 2, but Fable compile times weren't bad. OCaml is ridiculously fast.
- OCaml has an interesting module system that's missing from F#.
- The toolchain feels a lot more lightweight than the whole .NET toolchain.
- I found interop with JS and OOP stuff much easier in Fable than in ReasonML
- Fable has a tool for converting TypeScript definitions to Fable definitions - though it hasn't always worked for me for more complex type definitions
- I prefer F#'s whitespace sensitive syntax over OCaml, but that's just personal preference
- F# type providers are pretty cool
- F# has an interactive mode, and you can write standalone scripts (eg. for devops)
- If you want to write a web backend in the same language, F# on .NET is much more mature in the web space than OCaml.