Krishna Tritya is the name for the 18th Tithi (lunar day). It's the 3rd Tithi of Krishnapaksha. It's nature is Bala Prada, which can be interpreted as "one that gives power". It's good for following works: first feeding; Upanayana, Deva Pratishta, Manglik works; tonsure, cutting one's hair, nails, shaving; worshipping to something related to water (Apas, Jala); painting, singing, art; education; entering into house; jewellery; works related to animals (horses, elephants), conveyance.
Gauri is Devata which rules Tritiya (referring to Muhurta Chintamani and Poorva Kalamrita). Gauri can be translated as "fair-complexioned". It's another name of Parvati, a spouse of Shiva, who's considered to be "a Mother of All", "the one of Mountains" and all other female deities are considered to be her reincarnations or manifestations. As per Brihat Samhita, Krishna Tritiya is ruled by Hari (Vishnu) and all actions done in respect of Vishnu (maintenance, supportive works) are acceptable during this day.
Being Jaya (that can be translated as "victory" or "advancement") and ruled by Mangala (Mars), 3rd Tithi of Krishnapaksha is suitable for following works or actions: weapon making; wars; training in army, collection of army, any matters related to power or strength; travelling; festival; medicine; trades; house matters; flags & conch-shell.
During the vedic month Shravana Krishna Tritya becomes Maasa Shoonya Tithi, which is considered inauspicious in succeeding in works started during this Tithi.
Lagna (ascendant or rising sign) during Krishna Tritya becomes Tithi Shoonya (Lagna which is ruined by Tithi) for times when it is Makara (Capricorn) or Simha (Leo). All Uttara Nakshatras (Phalguni, Ashadha, Bhadrapada) also become Tithi Shoonya if they fall on Krishna Tritya. It's considered that all auspicious works done during Tithi Shoonya periods will be destroyed.