Frisk had known about Undyne's crush on Alphys for a bit, and had known about Alphys' mutual crush on Undyne for as long as she'd known her. His initial plan had been not to interfere so it happened naturally, but now that he'd been sent back in time, and could see quite clearly that the two of them were in need of assistance, he decided to lend a helping hand. It started with delivering Undyne's love letter to Alphys, as she couldn't do it herself.
Alphys mistakenly believed the letter to be from Frisk though, and wound up, rather indignantly, on a pity date with Alphys. Frisk realized he could put this to his advantage though, clearing out a section of Waterfall for the 'date', which the location of which he let slip to Undyne.
When Undyne showed up, Frisk bolted out of the area, or at least to where the two monster girls could no longer see him and assumed they were alone. Awkward standing around became talking, then confessing, then accepting, and finally, kissing.
Frisk grinned as he watched the two monster girls kiss. It was slow and gentle at first, both of them slightly nervous about it. Soon Undyne's natural aggressiveness kicked in though, and she picked up the pace, her hands trailing over Alphys' body, pulling her closer as she deepened the kiss, tongue pushing into her mouth. Alphys shuddered, still nervous, but too excited to resist Undyne's advances.
Undyne's hands traced down her back, finding and groping her round ass. Alphys moaned into the kiss. They kept going, Frisk getting a little worked up as they continued, both slowly stripping out of their clothing.
Frisk realized after the fact that his presence, his lust magic was probably what was jump starting their relationship, forcing their date directly to the lustful stage; not that either of them had needed much pushing in that direction.
As the monster girls continued, they paused, having heard frisk's gentle moaning as he watched them. Frisk froze when he realized he'd been heard, wondering if he could run before Undyne started throwing spears again, or if he was already stuck.
Thankfully, Undyne didn't respond with hostility, grinning as she beckoned Frisk over to her and Alphys. Frisk grinned, his excitement only intensifying the lust magic and he walked over to the new couple. Alphys was visibly nervous about Frisk joining them, but between the lust and the excitement in general, she decided to just go for it, much to Undyne and Frisk's delight.
Undyne pulled Alphys back into another kiss, then lowered it across the drake girl's neck, then down across her chest and towards her crotch. Alphys moaned softly, already growing wet just from the thought of Undyne's tongue against her pussy, let alone the actual feeling of it. She squeezed in surprise when she felt a second tongue though, Frisk's teasing between her cheeks and licking around the outside of her other hole.
Her body was tense, but it wanted it too much for her to stop them as Undyne and Frisk's tongue both entered her on either sides, both licking rapidly inside of her. Alphys moaned loud enough to alert any monsters in the entire area, though Frisk had already cleared the place pout for specifically that reason.
The yellow drake girl's hips moved reflexively for more, bucking back and forth against both of their faces. Undyne's hand moved up to toy with Alphys' clit as Frisk moved his hands to grope her round rump, the combined effort of them playing with her body pushing Alphys over the edge quickly. Alphys groaned as her juices washed over Undyne's face.
She licked her lips, the Alphys' so she could taste her own juices. Alphys blushed deeply at this, but it didn't stop her from licking her lips to taste it. Undyne grinned, moving Alphys onto her back as Frisk got behind them. The fish girl lined up her member, harder now than Frisk had ever seen it, to Alphys' eager, wet pussy.
Both of them moaned in pleasure as Undyne began pushing into Alphys, Frisk moving his head down to lick at Undyne's. The movement of her hips started slow, but eating the royal guard girl got more difficult the more aggressive Undyne's thrusts into Alphys got. Frisk didn't mind, enjoying the challenge.
He held onto her hips, tongue working diligently in her pussy while his fingers pumped into her other hole, four fingers working inside her ass and trying to match the pace she was thrusting into Alphys. With how eager the both of them were for this, neither of them lasted long despite Undyne's proven endurance.
Alphys reached it first, screaming out Undyne's name as her pussy clamped around her rod, squeezing it hard as her juices poured over it. Undyne grunted, continuing to hammer into Alphys for another full minute before she went over as well, moaning Alphys' name as she came, pumping warm seed deep into her.
Both of them panted, but Frisk's magic prevented them from being worn out too had enough in them for one more round, and now that Alphys had loosened up enough for the magic to get her to be aggressive, both of them wanted Frisk as well.
They pulled the femboy between him, Alphys forcing his head between her legs, locking them around him so he couldn't move away. Frisk's face was ground against the wet slit of Alphys' pussy for a few moments from her movement before he managed to push his tongue inside and begin eating her properly, his tongue vibrating with moans as Undyne's spear found its next target, penetrating Frisk's ass roughly and fucking him against her new girlfriend.
Frisk suspected it would be quite awhile before any of the three of them left Waterfall. He wasn't sure how exactly this would change the timeline, or even if it would at all, but he was already thankful for the reset Flowey had allowed.