Really different products.
I have used all 3 and I keep going / coming back to Whimsical.
I wish that Whimsical would have a few more features - they only need a few.
Mural - is really good for web meetings - it’s a real replacement for a basic whiteboard. It’s got dynamic polls and voting. It’s also strong on allowing all users to enter and view text. I haven’t looked too hard to see if there’s flowcharts or wireframes.
Miro - it’s a strong competitor. It’s better at dealing with and embedding images. It’s also more of a platform with extensions. So if you need to add a wireframe, it’s done with an extension. From a strategy point of view, that’s great - it is the one tool that you could do everything with. The problem is that it can be really slow to use, save etc.
Here’s a good review of Miro vs Whimsical