Katakana is used for special sounds, emphasizes of feelings, foreign words (words that were borrowed from other languages) etc. Kanji is used in varies nouns, verbs and adjectives where it is applicable. Most of those word has kanji. Originally, in ancient Japanese writings, almost every words is a Kanji. Hiragana is kind of like the alphabet in english. Essentially it is the bone of the Japanese language nowadays. You will see the most hiragana then kanji or katakana in a sentence. The form of hiragana was developed from Kanji. It depends on what you're trying to say in a sentence to use Hiragana , katakana or Kanji. You can write the whole sentence in Hiragana, tho that would be difficult to read and understand. Most sentences employ the use of all 3 character system. From the most commonly used being Hiragana > Kanji > Katakana.

Hiragana: native Japanese sounds. Commonly grammatic particles. Katakana: non-native sounds. Ice cream is written as aisu kuriimu in Katakana. Kanji: Chinese characters that act as concreate words. Nouns, verbs, adjectives, ect.