You don't need rogero downgrader when you're on 3.55 OFW/CFW, just install Rogero 3.55 v3.7 or Kmeaw 3.55 and remember to QA Flag when you're on 3.55 CFW as Kilam said earlier because that will help you downgrade your console with less problems and without having to use the E3 Flasher again if you decide to go back to a lower CFW. After you QA Flag, you should install a CEX CFW like Habib, FERROX, or Rebug Firmware ( I recommend Rebug) and then spoof to the latest CFW version to play online. If you're going to mod games online, I recommend converting your PS3 to DEX, there are many tutorials on YouTube that show you how to convert to from CEX to DEX or from DEX to CEX. If you convert to DEX you cannot install CEX firmware, and if you're on CEX you can only install CEX firmware so always remember that.

But how is the 3.55 firmware installed. Is it from the flashing process with the e3 or is it installed after the use of the e3?

I think that it is installed after the use of the E3, but I may be wrong.

That's the only part that I don't understand other then that I understand everything else