三さん次じ

• 第だい三さんsense 1

  第三 is the general ordinal 'third' used before nouns, while 三次 is reserved for stages, rounds, or technical orders such as 三次選考 or 三次方程式.

• 立りっ方ぽうsense 3

  立方 corresponds to 'cubic' in measurement or volume contexts, whereas 三次 specifically denotes mathematical degree or order three, as in 三次関数.

• 三さん次じ会かい

  after-afterparty; third party of the night

• 第だい三さん次じ

  the third; tertiary

• 三さん次じ元げん映えい画が

  3D movie

• 三さん次じ方ほう程てい式しき

  cubic equation

• 三さん次じの多た項こう式しき

  cubic polynomial; third order polynomial

• 三さん次じ元げんプリンター

  3D printer

• 三さん次じ

  Standard kanji spelling for 'third,' 'tertiary,' and mathematical 'cubic/third-order' uses.

• ３さん時じ

  Attested form, but in ordinary Japanese ３時 is normally understood as '3 o'clock,' a different meaning not covered by the senses below; 三次 is the clear spelling for this word.

A Sino-Japanese compound of 三 'three' and 次 'order, sequence, next,' giving the sense of 'third' or 'third order.' This is a component explanation, not a claim about the deeper historical origin of the characters.