In Threshold, the titular pyramid claims the lives of people in a particular pattern. Tirzah asks Boaz about this:
One day I heard two Magi briefly mention the numbers one, three, five, seven, eleven. They are another progression, perhaps.
Boaz tells her:
There is only one thing those numbers have in common. [...] They are all incomposite numbers, except the One, of course, which exists outside and beyond the others. [...] Incomposite numbers are those which cannot be factored - they cannot be divided except by themselves or by the One. They are thus indivisible.
I believe the "incomposite numbers" are what we would call prime numbers (i.e. numbers that aren't in the set of composite numbers). However, 2 is the first prime number, not 3, which is the second.
Most of the people killed by Threshold aren't important characters in the story - they're just there to make up the numbers. Not all of the events themselves are important, either - some simply increase the tension and move us towards the larger and larger numbers that are coming. It didn't seem like killing one extra slave (to start with two) or having another pair of deaths (to continue with two after "the One") would make a significant difference to the story.
Douglass appears to be quite careful with other mathematical aspects of this story, so it doesn't seem like missing 2 would be a simple error. Is there some explanation as to why the sequence didn't either start with two or continue with two after the special "One"?
(I'm fine with an in-universe explanation, if there's one I missed, or an out-of-universe one if Douglass has commented on this somewhere).