Under the terms of the license agreement, DocuWare services may only be installed once (or in the case of an Enterprise license - twice).
It is an accepted practice that the production license may also be used in a test environment as long as it is a test environment and not used in any way for production. Since you specifically ask about a third installation of the same license then the only answer is no. I would suggest you use the test license provided as Peters Engineering.
Senior Director Support Americas