cross-platform development for Windows, Mac and Linux

I want to develop a new system in the area of infrastructure/GIS. The system should be available under Windows, Mac and Linux. As I am starting from scratch, I have free choice of tools to use. Database Layer will probably be PostgreSQL/Postgis. It should work with Briscad alone (no need for Autocad-compatibility) Which platform for Briscad development fits best for cross-platform-development with minimal need to write code 3 times?