Has it really been a year? I remember when your husband brought those kitties home to you! You could probably use the search function and find quite a few threads on intro's, but my suggestion is to keep telling T&T that they are the top cats, that the new kitty is for them but is not replacing them, and that you love them endlessly. Do everything for them first (food, grooming, treats) - believe me, I know the tendency will be to pamper the new arrival, but the resident cats will sometimes believe themselves displaced.
Pam Johnson-Bennett is a cat behavorist with several books out - I bet your local library will have at least one of her books, and she's got a lot of great advice, IMO, regarding multi-cat households.
I had luck with feeding my two near each other so they thought good things happened near each other, and I also had a feliway diffuser going! I did do the separate room deal, but that closed door really upset my resident cat. After two days, I just let them both out into the house, and stayed home that Saturday to distract anyone who seemed overly intent on chasing the other one. Over the next week or two, there was chasing, hissing, and a couple of yowling sessions, but it ended well. I did keep toys handy at all times to sort of toss in their direction if anyone started staring at each other preparatory to a chase.
I'd definitely add at least one more litter box, and if you can add a cat tree or other way for each cat to have their own place to go, that's nice.
I'd say just try to give yourselves all some time - it's definitely nervewracking for you when it's not all joy and peace at first. But, it's important for you and your husband to radiate 'love and calm', because the cats will pick up the stress and then get themselves into even more of a tizzy over the new changes. And good luck to you all - it'll all feel worthwhile the first time you see them calmly looking out the window together, or cuddling up to you and your husband on a cold evening.