First, I'm a little unfamiliar with this problem--why this bad:
"our peers competing to put forward Who Is Hiring threads by submitting them increasingly prematurely"
Maybe this suggests that &#62;1 submission a month is better?
Finally, perhaps it'd be best to stagger the three post types throughout the month so they don't compete with each other for attention?
Ask HN: Should we agree that this account will submit all Who is Hiring posts?