The order doesn't matter, but you prob want to be available for comments/replies in case the launch goes particularly well, so do them 1 at a time.<p>Twitter feels pretty irrelevant to me, unless you have a massive amount of followers. Don't use Twitter to promote PH or the others, because you'll get banned or penalized.<p>I'd generally launch first on the platform where you have more experience, if any. Unless you want/have to test your launch, in which case choose the 2nd and use any learnings for the 1st.
It really doesn't matter - your launch should primarily be to your audience. The platforms are a nice bonus, but shouldn't be the core focus of your launch.