So our other cat 3 years old Oscar is also unimpressed by the 2 new arrivals. He's not been aggressive with them, if they've got too close he hisses and if that doesn't work they get the deep growl, that does the trick and they leg it. He'll come around soon, we did feed them next to each other yesterday and everything was fine until Ernie on finishing his decided he wanted in on Oscar's as well. They went nose to nose for a sniff but then the growl came and he legged it. I think in a week or two they'll have him playing, he loves to play with Milo next door who is a year younger than him, they are constantly rough and tumbling.
That kitten looks super sweet
Our cats are a similar age to yours at almost 4 years old (a rescue that we've only had for 4 months) and the kitten sisters are 4 months old. There's been no fur flying but certainly a lot of hissing from the first resistant alien, in fact she's not even got as close as a meter to them yet. As soon as she hisses the other two scarper.
I'm not sure how long we should give it before realising it won't work.