Furnace is a multi-chip tracker. The import into AT3 is only basic: subsongs and patterns – without effects but volume – should be imported correctly.
Instrument import is farily basic. ADSR/LFO/samples are not supported, as well as non-AY/YM instruments, and will only result in a small “beep” marker.
The goal of the import is only for you to convert your Megadrive (or whatever) score into AT3: it’s up to you to re-create the instruments.
Please note that only a subset of versions of Furnace is supported. The format has *many* versions – and is one of the most convoluted and illogical format ever. The Furnace website mentions a new format (“240”) but the current version does not produce it. If you need such format, please contact me and I will make the upgrade. Also, the minimum version supported is “181”. If you want to import such an old format, load the music into Furnace and save it again to update it. Then import it into AT3.