The Ender 3 firmware caps the MAXTEMP setting at 260, you need to reflash the firmware to go above. Iirc the Ender 3 V2 firmware can go higher and I think you can flash that? Double-check so you don't brick your printer.
Do at your own risk though, I wouldn't recommend the 300-degree mod on an Ender unless you swap the entire tool head for a direct drive system. I already had two friends who destroyed their printer and almost had a house fire doing the Ender 3 300-degree mod lol. There are a lot of issues with the PTFE tube softening or even downright melting in my friend's incident. My theory is the stock PTFE tube with the Ender isn't high quality, and there must not be the best heat isolation.
