临江仙 rhyming and translation of phrase
前后阕各三平韵，一韵到底

You are exactly right. This means both the first stanza (前阙 or 上阕) and the second stanza (后阙 or 下阙) has three rhymes, each of them a level tone, and the entire poem uses the exact same rhyming set.
I think the main confusion is that ci are not supposed to rhyme in their current dialects: while the poem you quoted (杨慎，《临江仙·滚滚长江东逝水》) is written in the Ming dynasty, most Ming (and Qing) scholars regard Song dynasty rhyming course as the one they should follow. So Yang Shen's rhyming syllables, 雄,空,红,风,逢,中, are all in Group I of the Cilin Zhengyun, which is a Qing dynasty conclusion of rhyming courses in ci, and therefore are considered to rhyme.
That is, characters belonging to the same rhyme group in the CZ are considered to rhyme in ci (from the Qing dynasty onwards), even if the modern pronunciation is different; and vice versa. For example: 风 rhymes with 东 (both in Group I) and not with 争, 蒸, 增 (all in group XI).
Also, tones. Level tones (平声) are not supposed to rhyme with unlevel tones (仄声), but when ci transitions into qu, this requirement becomes loosened. Nonetheless, in the older cipai, you don't even get the chance to rhyme level tones with unlevel ones, for the tone requirement for the cipai itself excludes this.
P.S. If you want to try your hand on writing ci but do not want to look through old rhyme books, then you are perfectly allowed to write in the 中华新韵.