We kinda think Miley and Kaitlynn are inviting Brody Jenner into a threesome ... if they haven't done the deed already!

Miss Cyrus and Miss Carter are being plenty flirtatious and overly-friendly with Kait's ex Brody on social media and now Miley's even using him to promote a line of legal marijuana she's invested in! The new couple sent Jenner a weed bouquet for his 36th birthday and he showed it off on his Instagram Story, tagging Lowell Herb Co, the cannabis company Miley has $$$ in.

"WEED like to wish you a Happy Birthday! Love you! Miley + Kaitlynn,” the card read. Body says, "I have never seen one of these in my entire life. Oh my God. That’s the kush too. Happy birthday to me!" Hey, you've gotta give props to Miley for using her split from Liam Hemsworth and subsequent hookup with Kaitlyn -- who was recently split from Brody -- for some pretty good PR!

Meanwhile, further evidence of Miley's fond memories of her trip in Italy where cameras first spotted she and Kaitlynn making out on board a boat on Lake Como, her tattoo artist Dr. Woo posted a snap of the singer's tribute to the vacay. Miley got ink of the Visconti of Milan coat of arms -- a snake eating a person, framed in a baroque pattern. It's not unlike the matching Vegemite tattoo she copied from Liam, but we see how that went ... paging Dr. Tattoff!