You can download the latest software from Märklin's website, or follow the instructions on your CS3 to update to the latest version.
RailControl Pro currently supports Märklin CS3 software versions 2.4 and higher, including version 2.6. In a future version of RailControl Pro, support for 2.4 will be dropped and 2.5 will be required.
Related content: CS3 software release notes