I'm not sure if you're being literal about the sandwich or if it's meant in the horny way, so I'll answer both options.

Actual sandwich: it'd probably be one of those soft breads that have blueberries or some other type of berries inside, where at the first bite it's soft and unassuming on your tongue, until you bite one of the hidden berries and bam. Explosion of flavor (ofc the bread would be both Shigeo and Mob). Reigen would be a nice few slices of Serrano ham, lettuce, and maybe some gouda or Edam cheese. The condiments could include either french mustard or chipotle, since their relationship would be a little spicy but not eclipsing the sweetness of the core (both the bread and the ham).

For the horny version: Shigeo would be the driving force here... At the beginning, anyway. He'd want to shut Reigen up from all his pent up resentment, so he'd probably make him deepthroat him in the sandwich. Mob would feel possessive of his shishou, so of course he'd be the one to "claim" him from behind, effectively spit roasting Reigen between the both of them.

I think at one point though, Reigen might take the reigns again and remind both of them why he's their shishou after all. This meaning even if he bottoms he'd guide them and occasionally be more dominant etc.

I need a ShigeMobRei sandwich with urgency punching the wall