The hardest game I've ever made! Try now!
I see that you only fill 3 boxes. You need to fill all boxes (except the middle) to get the board accepted. Each word must be a valid word, like:
...which works.
Do you mean the hardest to make or that it's the most challenging?
It's the most challenging game, not hard to make. Just a challenging game.
I see.
:(
The reason why this happens is because I use the dictionary from Google's top 10,000 words rather than the full word list. Not all 3-letter words are in the list, so that's why this happens. Sorry for missing some words... 
You can use this api to see if a word is in the disctionary.
The great thing about this api, is that you can just use it in the url block without bothering with a cors proxy (seriously, try it out).
This is the result you would get if a word exists
And this is the result you get if the word does not exist.
Now yeah, I know it gives way more information than you need, but at least it's free and works in snap. The biggest problem is that it's slow.