open source C/C++ interpreter for PostScript 3

There is no such Open Source C/C++  interpreter

Perhaps you can find Third party postscript interpreter  but it is not free and they don't deliver the source code  :-)

Cheers

Adam

"PostScript 3" is a trademark of Adobe Systems, Inc., and can only be used for their licensed products. However, PostScript languagelevel 3 can be freely implemented. By GhostScript for example. There is not a large choice that meets your specification. GhostScript is free to use within the terms of its license, which you should check very carefully, for example you may be required to make all other source code you write available under the same license.