You have to use both kana and kanji, and it depends on the word. Usually, kanji is for nouns, adjectives, and verbs, while katakana is for loanwords and hiragana is for grammar and native Japanese words. Let’s see an example: to drink = 飲む = nomu has the kanji at the beginning as the word’s “stem” (that never changes) and the む that often changes when you conjugate the verb. So for example, not drink = 飲まない (nomanai), drank = 飲んだ (nonda). Let’s look at the following sentence: John is walking to school. ジョンは学校に歩いている。 jon wa gakkō ni aruite iru. It has both kana and kanji John (ジョン), being a loanword, is written in katakana. School (学校) and walk (歩) are written in kanji, since school is a noun and walk is a verb. The test is written in hiragana - the は tells us that John is the subject in the sentence, the に functions as a “to”, because he is walking to school, and the いている at the end tells me the verb (to walk) happens right now (similar to present progressive in English).

So the short answer, yes, we use all three in a sentence. People will probably be able to understand your meaning if you only write in kana, but in general you have to use all three.