Silly me, forgetting that nothing about women's clothing is standard size.  I bought these mid-pregnancy, planning to use them so that I could continue to wear my bras from early pregnancy longer, as my ribcage continues to expand.  Well, my 3 hook bras (38Cs) have a MUCH narrower 3 hook closure than these extenders.  These extenders are 2 and 1/4 inches wide, and each hook is spaced 3/4 inch.
I made them work since I can hook the top and bottom of my bra to the first two spaces on the extender.  It just didn't seem to be worth the trouble to send them back since I won't need them for that long before I transition into nursing bras.
So, where is says "fits into most 3 hook bras" ..... yeah, some actual measurements would have been nice!