So three was never going to be a crowd when Busy Philipps joined the couple on holiday in Cancun, Mexico, this week.
Jason, 32, and Michelle, 32, looked the picture of happiness as they wandered along the white sands while immersed in conversation.
And Jason seemed to be tickled pink as the group made their way down the beach, with the star giggling away as he soaked up the sunshine.
Meanwhile, Michelle's best pal Busy, 33, who is expecting her second child with screenwriter husband Marc Silverstein, showed off her cute baby bump in a bright red bikini.
With her blonde locks scraped into a loose bun on top of her head and a pair of sunglasses sheidling her eyes, the actress seemed to be positively radiating with a healthy glow.
It's no surprise the trio were enjoying a sunshine break together, as Jason and Michelle have Busy to thank for their relationship after she reportedly introduced the two in early 2012.
Busy is also godmother to Matilda Rose, Michelle's seven-year-old daughter with late actor Heath Ledger.
Despite a few short-lived romances, Michelle - who won an Oscar nomination this year for her portrayal of Marilyn Monroe in My Week With Marilyn - remained largely single since splitting with Heath in September 2007, shortly before the star's tragic death.
Friends of Michelle said they had never seen her happier - while Jason told Parade magazine in March: 'I don't want to have a girlfriend for the sake of having a girlfriend. I have no desire to pick a random girl.
'I want the right girl. There are plenty of opportunities, but I would rather have the right one.'
Michelle and Jason recently moved to a sprawling loft overlooking the Manhattan skyline in Brooklyn, New York, and the actor has been playing an active role in Matilda's life.
Watch Michelle Williams' Surprise Cameo on BFF Busy Philipps' Cougar Town!
Michelle Williams and Busy Philipps are best friends who know how to keep a secret!
The former Dawson's Creek costars briefly reunited onscreen during Cougar Town's TBS debut on Tuesday, Jan. 8, when Williams made a surprise blink-and-you'll-miss-it cameo at the end of the season four premiere!
And guess what? Williams, a three-time Oscar nominee, is funny. Like really funny! "What what?!"
At the end of the episode, Cougar Town teased what fans can expect from the rest of the season, which jokingly included Williams' appearance as Laurie's (Philipps) foster sister, the one who loves cupcakes and teaches her the proper way to say "What what?!" Hm...sound familiar? It should, as Philipps once told us that she's love to have Williams on as her sister!
Philipps and Williams recently hit the beach with Williams' beau Jason Segel while vacationing in Cancun. Philipps, who is the godmother of Williams' daughter Matilda, is expecting her second child with screenwriter hubby Marc Silverstein. She announced the news via Twitter in December.
In its TBS debut (it originally aired on ABC), Cougar Town attracted 2.2 million viewers.
Cougar Town airs Tuesday at 8 p.m. on TBS.