What is the meaning of name "Usman" (the name of third Caliph)?

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

The word "Uthman" means the young bustard a large terrestrial bird, a young snake, or the snake itself. This is the opinion of al-Fairoz Abadi in his lexicon.
As you mentioned in your question, it is the name of the third of the well-guided Khalifa. Twenty companions of Allah's Messengers have the same name as stated by al-Fairoz abadi.
Allah knows best.