When writing a modern Sonnet is it acceptable to separate the 3 quatrains with white space?
Yes! Sonnets have one of the most prescribed structures of all types of poetry. Defined, sonnets contain 14 lines of iambic pentameter and follow one of two schools which differ slightly in rhyme scheme: ABAB, CDCD, EFEF, GG or ABAB BCBC CDCD EE. Of the two schools, the first given style traditionally reflects three different views on a particular topic of questions with the final couplet providing the answer as the speaker concludes. The second traditionally has presented a problem or scenario in the first eight lines and given a solution or analysis in the last six.
Note that I said the word "traditionally." Writers of poetry may often take a traditional style, in this case the styles of Pre-Renaissance and Renaissance-era poets and create new twists. Seamus Heaney would be one example.
Basically, I would say that if you, a modern poet, are presenting three different aspects to a situation or problem, it is certainly fine for you to separate the stanzas with white space. This would enable the form to provide the reader with a visual clue when a new aspect begins.