ATmega fuses for external crystal oscillators greater then 8 MHz
Based on the table - if CKOPT is 0 in the next column CKSEL3..1 lists all 3 values (101, 110 and 111) so that suggests they are equivalent, and that is used for frequencies over 1 MHz (and up to 16MHz as far as I can see that's maximum supported).

As per DataSheet for CKSEL = 0 oscillator has greater output swing thus supports higher frequency. CKSEL = 1 offers smaller frequency range but significantly lessens power consumption and this is probably why you have support for 0.9-3MHz to have even smaller consumption.

So my interpretation of this would be: CKOPT = 1 and CKSEL3..1 = 111 for frequencies 3-8MHz if you can help it (because of power consumption), and CKOPT = 0 and CKSEL3..1 = 111 for frequencies 8+MHz - 16MHz