There are four Find-Your-Fate books in The Three Investigators "keyhole" series. They can be found in the regular paperback trade edition with the same measurements as the rest of the "keyhole" books and they can each be found in a "Reinforced Library Binding" (similar to the GLB logo but no elephant) hardbound format which have slightly larger dimensions than the paperbacks. Unlike the rest of the "keyhole" series, these books are illustrated throughout. Each of the four books has 124 pages. The first two books are quite easy to acquire but the second two will require some effort to locate. The hardbound editions are all very difficult to find especially in nice condition.
The books are:
#1 The Case of the Weeping Coffin - by Megan & H. William Stine, illustrated by John Hulsey, 1985.
#2 The Case of the Dancing Dinosaur - by Rose Estes, illustrated by Vincente, 1985.
#7 The Case of the House of Horrors - by Megan & H. William Stine, illustrated by Aristides Ruiz, 1986.
#8 The Case of the Savage Statue - by M.V. Carey, illustrated by Tom Leonard, 1987.
Do not be confused by the fact that the books are numbered 1, 2, 7, and 8 on the spines. There are four additional books in the Random House Find-Your-Fate series but they DO NOT feature the Three Investigators.
Christian Andersch has created "graphical solution trees" for each of the four T3I FYF books. You can view these by clicking on the cover of each book, above.