They started taking bets.
The three of them hadn't had the time to really notice anything between Jyn and Cassian before the battle on Scarif, but after… Well, let's just say that they were surprised that the two of them didn't burn down any of the rebel bases with the fire that was growing between them.
The first bet was simple. Who was going to confess first and when.
Chirrut guessed: "Cassian. Right after saving Jyn during a rough point in battle.".
Bodhi guessed: "Jyn, the second before they step into battle.".
Baze guessed: "Both, after K2 gets tired of them dancing around each other and announces it to them both.".
Baze won.
They were lucky enough to witness it. Jyn and Cassian had stepped off of different ships after a difficult battle and practically ran to each other, so visibly relieved to see the other one alive. K2 was stood beside them and after just a minute he made a noise that sounded like a frustrated sigh and loudly said: "Would the two of you finally admit your feelings for each other for the sake of what little sanity I have programmed in me.".
You could see the redness of their faces even from afar.
Jyn grabbed Cassian by his sleeve and dragged him off to where onlookers assumed was somewhere private. The pair returned five minutes later, their faces still a little flushed and the tiniest of smiles on their lips.
They bet on little things too. Like who was going to initiate the first kiss (Bodhi won, it was Jyn before Cassian headed off to a particularly dangerous fight), who said 'I love you' first (Baze won, it was Cassian after Jyn was sent to the medbay due to a serious injury), when they would start sharing bunks (Baze won again, it was immediately after Jyn was discharged from the medbay).
The next big bet happened when they were hopefully near the end of the war. Who was going to propose and when.
While neither Cassian nor Jyn had said anything that would suggest that they would take this next, and big, step in their relationship, the three of them could tell it was going to eventually happen.
Chirrut guessed: "Cassian during a battle.".
Bodhi guessed: "Cassian after a battle.".
Baze guessed: "Jyn after Cassian is wounded.".
Chirrut won.
It was during the final battle on Endor and unfortunately they didn't get to see it (as they were busy fighting) but after insisting, the couple retold the story when the five of them were alone. They were huddled behind a fallen tree to avoid the blasters from some stormtroopers, trying to fire back as best they could, when Cassian popped the question. He laughed and said the look of shock on Jyn's face was hilarious, earning him a soft whack on his arm from her. He continued and said that Jyn yelled at him, asking if now was the best time to ask that. He had said that he couldn't think of a better time.
In the end, she had of course said yes and they even had a moment to celebrate with a kiss.
They didn't bother making a bet on what kind of wedding the pair would have as they all agreed that it would be a very small affair (they were right) but they did make a bet on what kind of dress Jyn would choose, if she wore a dress at all.
Chirrut won.
It was a simple white dress.
It was the final bet. There was no way Jyn and Cassian would have any other major (or minor) happenings in their relationship. The bet was if they would have any children.
Chirrut guessed: "Yes".
Bodhi guessed: "Yes, but through adoption.".
Baze guessed: "No.".
They figured neither of them would be in any hurry to have children, if they even wanted them, so they gave the bet a ten year time limit starting from when the couple left on their honeymoon. And every now and then they would ask K2 if he knew anything, like, were they trying or not, or at least talking about it. And every time the droid would flat out tell them 'No'.
None of them knew who would win the bet until sometime after the six year mark.
Jyn and Cassian had just returned from a quick trip to Jakku to buy some new parts for a couple of ships. There was a buzz around the base, people said that the pair brought a baby back with them and the same people began wondering what would happen to the child. Would General Organa adopt her? Would someone else?
Bodhi, Chirrut and Baze immediately headed to the medbay where Cassian and Jyn were there with General Organa and a doctor looking over the baby, checking to see if she was healthy.
The three of them bombarded the couple with questions of their own. But the only one they cared about having an answer for was: "Are you going to adopt her?" Bodhi asked, holding his breath just like Baze and Chirrut were, waiting as patiently as possible for the response.
Jyn and Cassian took one look at the giggling baby, then at each other before turning back to their friends, and said, "Yes.".
Bodhi nearly made the baby cry with how loud he screamed.