Intrigue in desert, attic, and train; The Turquoise Toad Mystery, by Georgess McHargue. New York: Delacorte Press. 139 pp. $9.95. (Ages 9-12.)

The Great Egyptian Heist, by Virginia Masterman-Smith. New York: Four Winds Press. 180 pp. $8.95. (Ages 9-12.) The Ring of Zoraya, by Harriet Graham. New York: Atheneum. 177 pp. $9.95. (Ages 9-12.)

Here are three action-packed mysteries to capture the imagination of young readers in the middle grades.

In ''The Turquoise Toad Mystery'' the prospect of spending Christmas vacation on an archaeological dig in Arizona thrills young Ben Pollock, a native New Yorker. Ben, who's interested in archaeology and photography, plans to put these skills to use. But soon after he arrives his attention is caught by a number of mysterious incidents. Crisscrossed tire marks in the middle of a dry creek bed miles from the nearest town, suspicious sounding CB messages, and deadly scorpions found in his tent make Ben pretty sure someone is trying to ruin the credibility of the dig as well as steal the valuable artifacts. So, instead of spending his vacation digging, Ben keeps busy trying to discover which archaeologist is part of a ring of thieves.

In ''The Great Egyptian Heist,'' Billy Beak's plan to search for his best friend, Tad, who has run away from home, is interrupted when another friend, Angel Wilson, daughter of a famous archaeologist, is sent home from Egypt. When Angel discovers Tad in her kitchen, Billy is upset because his rescue plans are ruined. But when Billy, Angel, and Tad discover a stash of stolen diamonds underneath the false bottom of Pharaoh Nikhamatumin's coffin, which is stored in Angel's attic, their adventures really begin.

There are plenty of suspects, among them the dancing Dutchman who has captured the heart of Angel's housekeeper and the mysterious Egyptian emir who offered to buy Angel for $2,000. But finding out who stole the diamonds and how Billy, Tad, and Angel catch the thief will entertain and surprise even the cleverest reader.

To ride on the Orient Express, the noblest train in the world, is something William, hero of ''The Ring of Zoraya,'' has often dreamed about. And now William is about to accompany his sister, Flora, and their guardian, Samuel, on the Orient Express from Paris to St. Petersburg, where Flora will audition for Pepita, the great Russian ballet master.

When they board the train, a young boy, claiming to be the Crown Prince of Slovadia, insists they share their compartment with him. At first William and Flora are put off by Michael's rude and demanding ways. But when Michael hands William the ring of Zoraya, William, Flora, and Samuel are caught up in a conspiracy that endangers their lives and catches up the reader in its mystery and excitement.