What does "3 of 15" people found this review helpful mean?

15 people read the review. 3 people found the review helpful, 12 people either had no opinion or didn’t find it helpful.
Same with the 0 out of 10 people. 10 people read the review, no one found it helpful or chose not to comment.

It means out of 15 people who read the review, only 3 found it helpful. That means if it stays this rate, 1 out of every 5 people will find it helpful. The statistic is only for those who voted.

The 15 is how many people evaluated whatever you are looking at. 3 people out of the 15 people who listened to it or watched found it helpful. It could mean more people watched it, but they used an equivalent fraction. For instance 30 people possibly participated and 6 found it helpful, but they stated it as 3 of 15. Similar to if 1 in 100 people are born with red hair, it does not mean only one person has read hair.

There’s no way to count how many read the review. I’d say it means 15 people rated the review, and of those, 3 said it was helpful. That means that the other 12 chose a different rating.

It means don’t ever rely on reviews, form your own opinion by experiencing that which is reviewed. lol

It means the same as:

Four out of five dentist surveyed recommend Crest™ as an effective decay-preventing dentifrice when used regularly in a comprehensive program of oral hygiene.

Only with different numbers.

Do you think so? I think they’re usually real numbers rather than ratios. “Four out of five” is a ratio; it’s 80%. But when they say that 189 out of 214 people found this review helpful, I’m pretty sure they’re not giving us a reduced fraction.

I agree with @jeruba even though I wrote basically what @CWOTUS said. I think most likely what the OP is looking at is actual numbers, not a ratio. But, the basic idea is the same. It does matter how many total people reviewed or took a survey or participated though. It matters for the validity of the opinion numbers. The more people who take ot, the more likely the information is worth something generally speaking.