Grilled Beef, Chicken, Shrimp, and Mushroom Skewers

Mince garlic and mash to a paste with salt using a large heavy knife. Transfer to a small bowl and whisk in lemon juice and pepper, then add oil in a stream, whisking until emulsified. Divide marinade among 4 (1- to 2-quart) bowls.
Cut steak into 20 to 25 (1/4-inch-thick) slices (about 4 inches long and 1 inch wide), then toss with marinade in one of bowls and thread each slice lengthwise onto a skewer.
Cut chicken breast halves diagonally into 20 to 25 (1/4-inch-thick) slices (about 4 inches long and 1 inch wide; if breasts are thin, hold knife at a 45-degree angle to get 1-inch-wide slices). Toss with marinade in another bowl and thread each slice lengthwise onto a skewer.
Toss shrimp with marinade in another bowl, then thread 2 shrimp onto each of 20 to 25 skewers.
Toss mushrooms with marinade in remaining bowl and thread 2 small mushrooms or 2 large halves onto each of 20 to 25 skewers.
Prepare grill for cooking over hot charcoal (high heat for gas); see "Grilling Procedure," below.
Grill skewers in batches on lightly oiled grill rack, covered only if using a gas grill, turning over once with tongs, until beef is medium-rare, about 3 minutes total; chicken is just cooked through, about 3 minutes; shrimp is just cooked through, about 2 minutes; and mushrooms are tender, about 4 minutes.
If using a charcoal grill, open vents on bottom of grill, then light charcoal. When charcoal turns grayish white (about 15 minutes from lighting), hold your hand 5 inches above grill rack to determine heat for charcoal as follows:
Hot: When you can hold your hand there for 1 to 2 seconds
Medium-hot: 3 to 4 seconds
Low: 5 to 6 seconds
If using a gas grill, preheat burners on high, covered, 10 minutes, then, if necessary, reduce to heat specified in recipe.