Poll Results

Total votes: 2043

"Marry", "Mary", and "merry": 794 (38.9%)
What? They're all completely different.: 763 (37.3%)
"Marry" and "Mary": 247 (12.1%)
"Mary" and "merry": 140 (6.9%)
"Marry" and "merry": 74 (3.6%)
"Marry", "Mary", "merry", and "Murray": 15 (0.7%)
"Merry" and "Murray": 4 (0.2%)
"Marry" and "Murray": 3 (0.1%)
"Marry", "Mary", and "Murray": 2 (0.1%)
"Mary", "merry", and "Murray": 1 (0.0%)
"Mary" and "Murray": 0 (0.0%)

Wow, talk about bimodal. It's a good bet that the people who picked the "marry", "Mary", and "merry" rhyme option grew up in the American West, Inland North, Midland, or Canada, while those who picked them all being different speak English with a Commonwealth accent. This is one of the best known vowel mergers in the English language, the other being the cot-caught merger which is also partially spread across North American accents.

Even though these are "well known" in linguistic circles, most people are unaware of the subtle differences in vowel sounds between regional accents to this degree.