I'm curious, what's the difference you're seeing between those three platforms for your use case?
Training for our specific pronunciation isn't available on Alexa. Our app is called Tali, pronounced "tally", and to get it to work now you have to say "Tay-lie". Whereas Google Actions can be trained with custom pronunciations.
I should also say that Dialogflow is magnificent.
I'm on the same boat right now, was asked to build a POC in Amazon Lex, an Alexa Skill, Microsoft LUIS, Wit.ai and Dialogflow.
So far I've gotten to LUIS and Lex/Alexa, I'm glad to read Dialogflow is better :) (had high hopes for it)