Attacking all three tiers will pull your product in different directions, require you to build expertise in multiple channels at once, communicate to three different types of customers at once, etc. It's better to focus on one tier of the market. 

Why can't you just focus on Market Product Fit? Because as we know from Product Channel Fit, products are built for channels. That means that as you lay out your Product hypotheses, you need to include your channel hypotheses as well.  

Why can't you just focus on Market Product Fit + Product Channel Fit? Because as we know from Channel Model Fit your model hypotheses influence your channel hypotheses.  

1. Evolving - Your market, product, channel and model are always evolving and changing.

2. Breaking - An element gets killed off. Typically this happens with the channel, as was the case with Pinterest.

when one of the fits changes, you need to revisit all of the fits to ensure growth.