Krillin hadn't exactly been getting along great with Goku. he was frustrated by how effortlessly Goku did everything that he himself had to try three times as hard to do. What was worse was how nice he tried to be about it, never really seeming to know he had done anything wrong. And Master Roshi's siding with him all the time was the icing on the extremely frustrating cake.
Krillin had taken to training more in secret after the others went to sleep. Maybe if he trained twice as hard for twice as long as Goku, then he would be able to catch up to and even surpass him in power. It made perfect sense to him, but it was a bit more difficult than he had been expecting.
The main issue was that off sleep. The first night he stayed up to train extra he didn't get any at all, and dropped like a stone during Roshi's appointed nap time. Even on nights when he did remember to sleep, it was never enough to keep up during the day, even with the afternoon nap they got.
Krillin very nearly caved and brought up his frustrations to Goku and Roshi, but by that point it felt like a sign of weakness and he couldn't make himself do it, and so he continued to train harder for longer, working himself into the ground until one night he simply collapsed, being found and woken by Goku early the next morning.
Realizing that he was fighting a losing battle, Krillin settled on actually sleeping normally and just training harder than Goku did during the day. After all, if Goku slept like a log and was still improving, so too could he.
But this to changed just a few nights later. Krillin woke up shortly after he had gone to sleep to the sound of Goku getting up leaving. 'So he is training at night to!' Krillin thought, kicking himself for ever stopping. He got up and began to follow Goku, intent on discovering what kind of training it was that kept Goku a few steps ahead of him at all times.
He followed Goku to Master Roshi's room, and paused angrily, wondering why Roshi wouldn't let him in on whatever training was happening. 'Does he think I'm not ready for it?' Krillin wondered, 'whatever Goku's doing in there, I can do it to!'
His thoughts on this changed however, when he started to hear moaning coming from the room. Krillin, confused by this, and thinking that they couldn't be doing what they thought he was doing, crept closer and peeked inside to see, confirming that they were in fact doing precisely what he had been doing.
Roshi and Goku had both undressed, though Roshi's strong, almost herculean body made it much less upsetting a sight than Krillin had been expecting, and Goku had straddled him. Both were still wearing their weighted turtle shells, and Goku lifted his hips, lining Roshi's stiff member up before lowering himself down to ride him.
Krillin had known that Goku was a cunt boy, as it was a fact he was always just a little too eager to share with him. Now, though, as he watched him moan, riding Master Roshi's massive member eagerly, Krillin blushed deeply as he realized that Goku may very well have been flirting with him to some extend, constantly mentioning his status as a C-boy in the hopes that Krillin would take the bait and mount him.
This knowledge, along with the sight of Goku's ass bouncing as he rode Roshi, caused a tent to form in Krillin's pants as he watched, unable to look away. He had to force himself to blink several times. Roshi thrust hard up into Goku, working more and more of his ki powered member into him with each movement until Goku was taking his entire cock and moaning happily about it."
M-Master Roshi," Goku groaned, "Sh-should we invite Krillin in with us?" He asked, making Krillin's blood run cold. Roshi stopped moving, looking over at the door and spotting Krillin. He grit his teeth in panic, though outwardly he seemed fine. He had been hoping to keep these sessions secret, and had thought that if they got out, it would be because Goku let it slip, not because he had forgotten to lock the damned door.
Not wanting to lose face, Roshi nodded, "Of course, he's got every right to join in on the training, no one's fault but his own that he hasn't until now." 'It is training?' Krillin thought, baffled as he walked inside the room. "You do want to get stronger don't you?" Roshi questioned, Krillin nodding, "O-of course!"
"Then come closer and get to it!" Roshi ordered, reaching around Goku and spreading the cheeks of his ass for Krillin. Krillin nodded, blushing deeply as he came over, lowering his pants and freeing his stiff rod, "Sh-shouldn't it be lubricated somehow before it goes in there?" He asked.
"Nonsense. You should be strong enough to get it inside with force alone, and Goku should be strong enough to endure it. Or have you two been slacking off?" Roshi questioned. "No sir!" Goku said, determined. He turned back to Krillin, "Let's show him we can do it!" He said with a confident smile.
Krillin nodded, lining up and thrusting into Goku's ass roughly. Both of them grunted as Krillin struggled to work his member deeper into him, pushing in inch by inch. But as it turned out, Roshi had been right. Krillin's built up strength allowed him to force his cock in to the base, and Goku took it all without once crying out in pain, even taking some pleasure in the roughness of it.
Roshi meanwhile just took pleasure in how tight Goku got around him, his pussy clenching at the new entry. "Now get those hips moving!" He ordered. Krillin held onto Goku's hips and began to thrust into him at a steady pace, Goku bucking his hips back and forth against both of their rods to push them both in and out of him.
"From now on," Roshi explained, "Your training will involve regular weighted sex. A minimum of three positions a day, and one with me assisting." "Y-Yes Master Roshi." They both moaned, starting to lose themselves in the pleasure. Roshi chuckled, 'well,' he thought, 'this is one way to help them get along better.'