hello, for 3x concetrate you should make up the working solution by diluting it 1+2 with water (1+2 dilution = 1:3 = 1/3 concentration). however, don't be afraid to prolong the development further, this developer is softly working. maybe you will have to add more Disodium phosphate to it for 18 years old film (10-20 g maybe, maybe more).
good luck.