First things first, vowel pronunciation in English varies widely between dialects. I'm going to answer as a native speaker from the US west coast.
Also first things first, eight and ate are homonyms, both phonemically as /et/. it is /ɪt/. eat is /it/.
However, the actual pronunciation of these words varies considerably depending on the surrounding words in the phrase. Let's assume the words are pronounced in isolation. Due to the word-initial vowel, they will be pronounced starting with a glottal stop [ʔ]. Due to the final unvoiced consonant /t/, they will be pronounced with a very short-duration vowel. (I'm using "short" to refer to the duration only.) And the final /t/ will also be pronounced as glottal stop [ʔ]. So the pronunciation will be [ʔĕʲʔ], [ʔɪ̆ʔ], and [ʔĭʔ].
As a result of the short-duration vowels and the glottal stops, these words provide a very difficult environment for distinguishing the vowels /e/, /ɪ/ and /i/. I'd recommend you first become expert in distinguishing these three vowels in words where the vowels are easier to hear, such as made, mid, mead, phonemically /med/, /mɪd/, /mid/, pronounced [meʲːd], [mɪd], [miːd], or sane, sin, seen, phonemically /sen/, /sɪn/, /sin/, pronounced [sẽʲːn], [sɪ̃n], [sĩːn].
All that said, the actual difference between these three vowels is in tongue positions resulting in different sounds. For /e/, the tongue starts in a high-mid front position and moves toward a high front position. For /ɪ/, the tongue starts in a near-high near-front position and may move toward a central position. For /i/, the tongue stays in a high front position.