Hi Josie! We had 2 cats, 1 8yr old Siberian, 1 3yr old Ragdoll) and recently adopted a cat from a Shelter (dsh 3yr old). It constantly meows (his voice is now so hoarse) and pounces on the other 2, terrifying them! How can I make it stop, the meowing and for them to get along? It's been 3 weeks!
It will most definitely take longer than 3 weeks, especially for your oldest cat. Show them that being together means treats, play and other good stuff. Reward every positive moment and eventually you'll get there :)