Going back to the Snape/Lovegood/Lupin dynamic here is how I think it happened
Severus and Remus never were truly friends in the regard that their friends bullied the other, but after the prank they started to just relax and mellow out and started to become study partners as Severus worked on making potions to help him
Remus started to catch feelings first but only because Severus doesn’t want to get his own feelings hurt and throws away any feelings he gets for anyone/thing in fear of losing it
Remus and Sev smoke and get a little flirty banter here and there tho
Next year they get put into a class with Xeno and Remus just like “fuck wait he’s hot.” And immediately has a crisis because he feels like he’s cheating on Severus when they aren’t even fucking together yet
Severus sits with Xeno and Xeno is already In love with Severus and curses his name same. But Xeno knows that Severus is interested in Remus and vice versa just because Xeno isn’t socially inept like these two (but is still bullied lightly for being a weirdo)
They some how get put into a group for an assignment for some bullshit class and the project is to basically make a mini coven to show that magic is stronger in numbers and as a bonding exercise between the different houses.
They do stupid little spells and stuff, but they all feel their magic tie between each other
All three of them are assuming the other two are dating.
To preform some ritual they all share a drink and Xeno makes a joke saying he’s happy he gets to place his lips as to where the others have shared and Remus and Sev are just red.
“Wait what do you mean shared-“ Remus later realizes that Xeno thinks he and Sev have kissed before, (they have but it was CPR)(Sev nearly killed Remus at least once due to wolfsbane trial and error)(because Remus didn’t tell him about an allergy)
Xeno is just like “???” And Severus is like “???” And now they are all like “???I thought you were with him?”
Xeno then asks them why not just all be together, Severus bails, scared of opening up (although he has been slowly since the assignment) and both Remus and Severus scared to not only be in a homosexual relationship but in a poly homosexual relationship, and Remus doesn’t know how to tell Xeno he’s a werewolf
After a minor yelling match between the three of them (mainly Severus because he’s an emotional agro dude) they all cry and say sure we can do a trial run.
The trial quickly turns into just being a full blown relationship. Xeno has so much love in him he is happy to give it to two people, Remus loves being a switch having partners who are mentally stimulating. Severus enjoys having two partners because he doesn’t feel guilty for spending hours doing weird little freak shit instead of spending time with his partners because they have each other, but also likes it because they respect him as a person.
Also it turns out it Xeno already knew he was a werewolf because 1. He’s a Raven claw he’s not fucking stupid and 2. He has a natural passion for magical creatures 3. He doesn’t know too much about potions, but he knew the wolfsbane plant in snapes cauldron when he saw it
First kisses (besides the allergic reaction) is Xeno with Remus, kissing him good night after walking him to the Tower. Remus Later kisses Severus when the potion was a confirmed success, Xeno kissed Severus after hearing about the success, again wishing him goodnight (the dude stayed up all night just worried sick that the potion would some how randomly have an adverse effect on Remus and hurt him)