"in triplicate" is a correct and usable phrase in written English. It is used to indicate three identical copies of something that must be completed. For example, "I need you to fill out the application form in triplicate and bring it to the office."
Stonehenge is in triplicate.
Submit poetry entries in triplicate.
Samples were analyzed in triplicate.
Determinations were made in triplicate.
(c) Growth curves in triplicate.
Experiments were repeated in triplicate.
One character appears in triplicate.
Experiments were performed in triplicate.
This was performed in triplicate.
Now Rosalie Downey worries in triplicate).
Nintendo's pink puffball hero returns in triplicate.
Ludwig does not simply clarify my doubts with English writing, it enlightens my writing with new possibilities
Simone Ivan Conte
Software Engineer at Adobe, UK