cat intro went downhill out of nowhere, need help!!

hi y’all! i have 3 cats at the moment, all of them are neutered males. the oldest is 2 years old and the youngest is 7 months. at the beginning of the month my roommate and i adopted a third, who is about 1.5 years old.

i’ve been through the introduction process before so i know the standard procedure and things were going well up until a few days into face to face introductions. the main concern is between our oldest and the new cat, our youngest has been reacting pretty normally so far.

initially, they were getting along much quicker than we had expected, so we decided to speed up the process a bit and i do think that’s where we went wrong. they went from being a little timid but overall getting along really well to suddenly being absolutely terrified of each other and i don’t know how to fix this.

the new cat tried to initiate play with our oldest and this freaked him out, and since then things have been horrible. he’ll barely even approach him through a barrier, and when they’re in the same room our oldest hides and gets aggressive if the new cat tries to go near him. i’ve never seen him like this and it’s really upsetting me.

we’ve decided to cease the face to face interactions and have them go no contact for a bit before restarting the whole process, as i’ve heard sometimes this can help.

it’s only been 2 weeks but i’m feeling really discouraged about the whole thing and would really like some advice on things i can do to help the situation. they don’t need to be best friends, i just need them to be able to peacefully co-exist together. thank you.

edit: we did get a feliway diffuser a few days ago and i am hoping that once that begins to take effect it will help