I think the whole thing is a ridiculous/crazy mock-satire over things to do with love, marriage and divorce, and the three weddings are dream fantasy sequences or silly takes on certain relationships that are forming in Season 3 from either a character's or the narrative's POV: Perhaps one is seen from PSH/Ah Mi or Dong-Mi's POV, one from Song Won's POV, and the last from Dong-Ma's.
Dong-Ma, because I think he seems to imply both to his brother in his discussions with him and through the season that he may have a thing for older mature or more independently-minded women (or women who aren't clingy and needy at least) and he appeared to me to have been taken aback when he saw SPY and Dong-Mi on the golf course. He'll probably see more of SPY through his brother's work. So at the very least, he may find himself comparing SPY with Ga Bin during Season 3 and discovering he prefers SPY and thus has that fantasy of marrying her over Ga Bin. But he needs to explain his about-turn over Ga Bin and wanting to marry her all of a sudden, because that desire to marry her came out of nowhere.
Song Won, because the realities of falling for an immature romantic idealist like PSH and having in-laws who only seem to care about the baby's sex/gender (and whether their son's spouse is a whore in bed, a housemaid in the house, a mother his child, and a non-gold digger or financial leech with her own job & career) are beginning rear their heads with the demands of a newborn baby on tow. PSH behaves like a child still tied to his mother's apron strings. But in comparison, nearly every woman's eyes on that show, Seo Ban represents the near perfectly flawless male to date and marry. He's mature, mysterious, manly, rich and doesn't have a hang-up over children that aren't his. So for Song Won, with whom SB will be having Chinese lesson with her baby on tow, SB will probably serve as a sounding board for all her woes, just she was for PSH in Season 1 and she fantasies what it will be like to be with him. I hesitate to call the dream fantasy Seo Ban's because he's not that way inclined to romantic fantasising. He appears to much of a realist for that. And the fact that neither Seo Ban nor Song Won were called "bride and groom" in that scene makes me think that this scene is less about them getting married to each other than it is fantasing what it would be like to marry someone who is like that in their personality & character. For Song Won, it will be a dependable mature man; for Seo Ban, it might be a woman who isn't after him and his money and doesn't have weird hang-ups.
And Ah Mi (or PSH), because the woman is plain silly and childish as PSH and her current family set-up with Yoo Shin does not appear to bear the kind of fruit she was hoping for. Yoo Shi still hankers after his ex-wife, his kid's possessed by his dead dad, and his step-mother for no discernible psychological reason keeps going gaga everytime she claps her eyes on her step-granddaughter. Somehow, it's orchestrated that Ah Mi and PSH meet one another in the course of Season 3, and because their personal lives are such a mess they're at pains to share a common pubescent teenage angst woe. PSH moans that post-natal Song Won no longer has time for him, making him his breakfast and their kid cries all the time wanting attention all the time; Ah Mi moans that everyone runs around the spoilt brat Ji-ra and Yoo Shin has no time for her anymore. In fact, this dream fantasy may not even be from either Ah Mi or PSH's POV; it's probably from Dong-Mi's POV in her desperation bid to finally get rid of Ah Mi out of Yoo Shin's life once and for all.
