I hate to disagree with Snakeeyes, however regarding the Centre Spead feature of the DSU is not required to be turned on for Movies. Centre Spread is for two channel sources where there may be a tendency for it to give undue emphasis to the Centre Speaker, pulling the image toward the centre. It does not hurt to turn it on and if you prefer it on then that is fine.
The Centre Spread option also has serveral hidden parameters including width, dimension and image. These can be adjusted via the web setup, <IP address of the Receiver>/Setup, mine for example is 192.168.1.100/setup. The values mentioned are in the Surround section, Sound_Program_Param sub section for SUR_Decode. It may also be possible to set these from Yamaha's AV Setup app. It is not as far as I am aware settable via the on screen menus.
Regarding your case of 3.0, it is a very good question. I cannot honestly say for sure. Most of the codecs are discrete channels for 5.1 or above. Stereo and Mono would obviously be unaffected. Pro logic, Neo or Neo 6 are actually matrixed Stereo where the information for the centre, rear channels is extracted, so therefore should still work normally. Standard Dolby Digital, True HD, DTS, DTS HD MA are discrete either 5.1 or 7.1. Dolby EX and DTS ES are special cases of the Standard 5.1 formats with matrixed surround rear channels extracted. These should make use of the available speaker channels and so if less than the expected "should" blend with the main Left and Right speakers as is done when there is no dedicated Subwoofer. The main LR speakers should be set to Large, by default when there is no Subwoofer present. The Centre can be either Large or Small, with the frequencies below the cutoff directed to the Left and Right if the Centre is small.
Others here maybe able to give a more definitive answer or Yamaha themselves.