осеннее, осенние and осеннии all are [ɐsʲenʲ:ɪ.ɪ] for me. (Reduced a, soft s, stressed e, soft long n, short i and short i again)
The difference can appear only when I pronounce them slowly, sound by sound.
осенняя is [ɐsʲenʲ:ɪjə], with the first я reduced to [ɪ]. But [ɐsʲenʲ:əjə] sounds good as well.