I was able to fix this issue when launching Creatures 3 after opening the machine.cfg file in C:\Users\"user"\Documents\Creatures\Creatures 3 and removing the space at the end of the \"creatures " \ section of the address of every line.

example machine.cfg

Before:

"Backgrounds Directory" "c:\\users\\rigel\\documents\\creatures \\Creatures 3\\Backgrounds\\"
"Body Data Directory" "c:\\users\\rigel\\documents\\creatures \\Creatures 3\\Body Data\\"
"Bootstrap Directory" "c:\\users\\rigel\\documents\\creatures \\Creatures 3\\Bootstrap\\"
"Catalogue Directory" "c:\\users\\rigel\\documents\\creatures \\Creatures 3\\Catalogue\\"
"Creature Database Directory" "c:\\users\\rigel\\documents\\creatures \\Creatures 3\\Creature Galleries\\"
"Display Type" 2

After:

"Backgrounds Directory" "c:\\users\\rigel\\documents\\creatures\\Creatures 3\\Backgrounds\\"
"Body Data Directory" "c:\\users\\rigel\\documents\\creatures\\Creatures 3\\Body Data\\"
"Bootstrap Directory" "c:\\users\\rigel\\documents\\creatures\\Creatures 3\\Bootstrap\\"
"Catalogue Directory" "c:\\users\\rigel\\documents\\creatures\\Creatures 3\\Catalogue\\"
"Creature Database Directory" "c:\\users\\rigel\\documents\\creatures\\Creatures 3\\Creature Galleries\\"
"Display Type" 2

Hope this helps