#!/bin/bash
mytext='\\[[123 one (/)\n\\[[456 two (/)\n\\[[789 three (/)'
myvar=456
awk -v myvar=$myvar 'BEGIN{FS=OFS="\\\\n"} {s="";
	for (i=1; i<=NF; i++) { 
		if ($i !~ myvar) {s = s (i==1 ? "" : OFS) $i;}
	}
} END{print s}' <<< "$mytext"