Interesting observation, and puzzling. A quick check shows Rahlf's edition of the LXX missing them but Brenton's edition has them. They are also in Jerome's Vulgate which is a good textual witness to antiquity. Also, see the corroborating information in this
link .
What's bothering about them missing is that Lamentations is arranged as an acrostic of the ordered 22-letter Hebrew alphabet, the verses 3:22-25 being the section of chapter 3 beginning with the letter ח (heth), the 8th letter of the 22, just about smack in the middle. Their absence, though witnessed in the most ancient extant manuscripts, leaves a hole that, I would think, begs to be filled.