Flashing a bootloader on Ender 3 without Arduino I just order myself an Ender 3 Pro which will come by the end of the week.
Before it arrives I want to be ready to flash a bootloader onto it.

I was wondering what other options there are to flashing except using an Arduino?

I have a bunch of ESP8266/ESP32 and a USB to TTL. Would it be possible to use these somehow instead of an Arduino to flash a bootloader to the Ender 3?
Or should I just go buy an Arduino?
What you need to is called a ICSP or ISP: in-circuit serial programmer or in-system programmer, which excludes the USB to TTL device you own.

I've never used an ESP8266 as ICSP but it seems there are some resources out there reporting it is possible.
If you want to go the easiest way probably you want to buy an Arduino and follow the tons of tutorials out there, if you are looking to save some money then you might get around buying an ICSP like the very well known USBASP (just Google for that).