I would not take for good fish that this works, as this is my first regex attempt, and don't have any means to test it atm.
ON *:TEXT:/www.twitch.tv\/^(x3pos)/i:#3pos:{
://i: the two slashes are just the start and end of regex, while the i at the end makes it case INsensitive
www.twitch.tv\/ the \ is to negate the / so it doesn't consider it as the regnex end slash
^(x3pos) I'm unsure if this is correct, but hopefully it will take it as "not x3pos"