#!/bin/bash
echo "WORD1 %WORD2% WORD3 some text WORD1 %WORD2% WORD3" | 
   sed 's/WORD1/й/g' |
   sed 's/WORD3/ч/g' |
   sed 's/й[^йч]*ч/й ч/g' |
   sed 's/й/WORD1/g' |
   sed 's/ч/WORD3/g'