You can use one of the free Samsung firmware update tools to download the Samsung Z3 SM-Z300H stock ROM. There are numerous firmware update tools for Samsung that have been developed on GitHub and XDA Developers that are used by thousands of people each day to download firmware files directly from Samsung’s servers. One of the reasons so many people on XDA in particular download firmware files is that they need the boot image from the latest firmware to get root access with Magisk. But there are many reasons people want firmware files, perhaps the most obvious being they want to update the software on their smartphones.