2 blades or 3 blades ???

No real question for the vast majority of glow fuel models. There really aren't any 3 blades readily available, and almost none that take an effort to get.

But look at IMAC. They have a very good choice of 3s and after the initial development process that happened when they were basically forced to try 3s, they discovered that 3s worked as good or better than 2s had. Look at the history of pattern flying. Same story.

In both stories, there was a learning curve. It's still going on, but basically there was a discovery that with some choice of props, there was more performance out of 3s than most expected. And they discovered that some engines were better with 3s than others had been with 2s. That's not surprising because engines had been refined over time to better suit 2s simply because that's all there were.

When there is little or no competition, there is no real proof.

I heard an IMAC guy at the field awhile back griping about how poorly his plane was flying. Turns out he'd busted his expensive (and only) 3 and had dug out his last, best 2. He was advised to try a 3, that 3s were more efficient. Say what!!!! Our hobby is really humorous sometimes.