Place the flank steak in a dish and drizzle with olive oil. Sprinkle with Cajun seasoning and rub into the meat. Place the mushrooms in the dish and sprinkle with the seasoning. Cover and refrigerate for an hour or so.
Place the chicken and shrimp in separate zip top baggies. Drizzle each with the olive oil and sprinkle with the Cajun seasoning. Seal and shake the bags to distribute the seasoning and oil. Place both in the refrigerator for 30 minutes or so.
Take the steak out of the refrigerator while preparing your grill. The steak should come to room temperature before you place it on the grill. Cut the steak into bite size chunks.
Cut the chicken into bite size chunks.
To skewer the meat, just slide the meat down onto the point. Place a mushroom on the skewer and then a piece of onion. Repeat until the skewer is filled. You can distribute the chicken and steak as you like.
I placed the shrimp on separate skewers by themselves to ensure they didn't get over cooked while the steak and chicken were getting done.
Season the skewered meats with salt and pepper if desired.
Place the beef and chicken skewers on the grill. Turn them a quarter about every 6 to 7 minutes. Check for done-ness and continue to cook if needed until the chicken has an internal temp of 170 degrees F.
For the shrimp skewers, just place them on the grill for 4 to 5 minutes on each side. Cook until the inside of the shrimp is white. The internal temp of shrimp should be 130 to 135 degrees F.