I've mined and placed over 500 lecterns and haven't seen Unbreaking III in the first trades. I've seen Unbreaking I and II but never III.
Comments 3
See wiki:
The enchantment is chosen randomly with equal chance of any enchantment type occurring (except for Soul Speed) and equal chance to get any level of the enchantment, meaning high-level enchantments are as likely as low-level enchantments. The price in emeralds depends on the enchantment level and "treasure" status. The possible values are 5–19 emeralds for Lvl I, 8–32 for Lvl II, 11–45 for Lvl III, 14–58 for Lvl IV, and 17–71 for Lvl V. For "treasure" enchantments the price is doubled. Note that the cost is capped to 64, meaning that for example Lvl V books truly range from 17–64 emeralds with costs at the upper end of the range being more common.
UNBRAKING IS NEVER IN THE FIRST TRADE I THINK