Easy. You have a large number of Mics (up to 22), User Mics (up to 8), Buses (Up to 16) and Outputs (Up to 16). You have a section called “Show Channels” at the top center of your SD3 Interface within the “Mixer” tab. You’ll notice at a glance how many of those are in use (for instance: “Mics: 8 out of 22”). Each button shows you to the Mics, User Mics, Buses and Outputs channels and leads you to them by pressing them – As a clue, the single mic channels are blue at the top. The user mic channels are grey. The bus channels are yellow, and the output channels are red. BUT there is another button which reads “Edit Visibility – Show All Channels”. By pressing there, You’ll be shown all the Mics, User Mics, Buses and Output channels. Those which have the “HIDE” button marked in blue at the top are UNUSED in your preset. You just have to toggle the “Hide” button for the channels (either single mics, buses, etc) you wish to see  as available in your mixer. Then you can press again in the “Edit Visibility” button and those channels which are still marked as “hide” will not be shown anymore.

So, you’ll have a few new mic channels, buses, etc available which you have “unhidden”, so to speak. Now it’s up to you what to do with each of them. Hidden single mics are assigned to different mics which were actually used when recording the samples for the kit, but had not been chosen to be heard, so you have not much choice there but to use the one you have toggled on – choose wisely and edit the “channel properties” so that the close mic used for the channel is right. As for “User Mics”, I think those are destined to be used as virtual mics for single shot samples you may want to add and be treated as a single instrument mic. As for buses, you can, for instance, rename your newly shown bus as, say, “snare comp”, and create a send from the single mic snare channels and set the level for each send to taste. Then, click on the “Effects”area for your new bus and add a Dynamics effects (one of the available compressors, for instance).

You can also, for instance, group single mics outputs into a bus. For instance, you might have three single mic channels for your kick drum: Kick in, Kick out and Kick sub. You can create a “Kick” bus and set the OUTPUT (bottom of your tracks) of those single mic channels to the “Kick” bus, so that it acts as a “Kick sub mixer”. – So, you just have to toggle the hidden mic channels, bus channels, output channels, etc by finding them within the “Edit Visibility – Show All Channels” option and then edit those to your taste.

Ohhh… Just found a video about “creating new channels in the Mixer”.

I’ve timestamped it:

/Robin Gardner