Can LSTM have a confidence score for each word predicted?
Couldn't you require your softmax output to exceed a threshold for the prediction or you call it "not confident"?

I have not tried word prediction, but the softmax gives you a value for each output, where all of those values sum to 1. So pick the column with the highest value as your predicted word and use that value to determine your confidence. (It may be fairly low, since there might be a small chance of lots of words and then larger chances of your most-common three.)