1st u must have beat the e4 and have both the ruby and saphire gym in the machine on isle 1

then walk around Kanto until u find either raikuo,Entei,or sucine (depends on starter)use masterball if u have it then trade it 2 ruby saphire emerald start new game new starter do the same start new game new starter do the same excpet this time keep it and trade the other 2 over and u will have all 3