Thanks. The two OCZ SSDs now appear in BIOS. What I did was connect the two OCZs first, and booted with one of them into Windows, then I shut down, connected the Samsung, and booted into Windows again. After that, I checked the BIOS and all three were showing and have continued to show in BIOS and F12 boot menu through several boots. Why the procedure actually resolves I don't know. Thanks, again.
Gigabyte GA-Z170-HD3 (rev.1) BIOS vF6
Intel Core i3-6100 3.7GHz
Samsung 850 Pro 256GB
OCZ Vertex 2 60GB
2nd OCZ Vertex 2 60GB
Samsung HD204UI 2TB
Avexir DDR4 8GB
Intel HD Graphics 530
Apevia 640 watts
Windows 7 Professional