By default, TurnKey will install to the first drive it finds. If you select manual partitioning, then you should be able to select the specific drive that you install the root system to. So long as you use the LVM option, then you can easily extend the filesystem to the additional drives later. If you already have the system installed, but without LVM, then unless you've done much on it, it's possibly easier to just reinstall. Although it should be possible to migrate to LVM without a clean install (although I've never done it and not 100% sure of the process).

Once you have installed (provided that you use LVM), then you can add the disks as "physical volumes" (PVs). Then you can extend the root filesystem to span across the additional drives as noted in this (old but still relevant) blog post. The blog is explicitly aimed at extending the root filesystem of a VM, however the LVM instructions apply equally to a system installed directly on bare metal.

Alternatively, if you only wish to use the additional drives for a specific filesystem location (rather than just adding it to the root storage), then you can create a new "volume group" (VG) and then "logical volume" (LV). You can create a filesystem on the new LV, then mount that to the desired filesystem location. To make it permanent, add the new LV to the fstab file as well.

I know I haven't provided a ton of specifics or detail, but hopefully that's enough to at least get you started. If you have any issues, please ask and I'll do my best to head you in the right direction.