As for English speaker IPA for English might be helpful:
[i] кіт, key
[ɪ] миша, six
й (at the end of the word and before consonants):
[i̯] гай, найшов, йти: coin
I'm not sure about last one: at least I pronounce "i" in coin totally differently.
You can just try to pronounce "й" at the end of word a little bit longer and more vowel-like comparing to "й" before vowel:
й (before vowels):
[j] йога yes
As for being understood: I've found 40454 words that have all three endings. It is usual for adjective to have all of those in different grammatical forms. For example: adjective "високий" (high/tall):
високий: single masculine grammatical gender
високі: plural adjective for high
високій: single locative case feminine grammatical gender
So it WILL be misleading, though from context it is probably possible to guess what you mean... There are other cases where words or their forms differ by these endings so it would be better to learn to distinguish them and pronounce them properly.