Stacey's is a clown that turns into a hot dog. Because...a clown that turned into a hot dog scared me as a little kid, and while it certainly wouldn't be my own boggart, Stacey really is kind of naïve and innocent and I didn't think her boggart would be something deep and symbolic, but rather just silly.
Sylvie's is a big fire. Possibly engulfing one of her family members or someone she loves although since she's not as close to her family this time around, maybe not. But definitely still a big fire. This stems from her stealing borrowing her brother's wand when she was a child and set a tree (and nearly her house) ablaze.
Marietta's boggart has always been a homeless person because her worst fear was that they wouldn't be able to pay the rent and get kicked out of their flat. Now I think things are a bit different with her jinx and knowing she'd been Obliviated. So maybe now it's *her* - homeless and obviously confused.