I love food questions. I did a lot of food culture research regarding heresy, which sounds out of left field, but what people had to give typically was food. That's how they'd support heretics (and Franciscans alike). What they have and what they give and how they give it reveals a lot.
The real answer is the Middle Ages consists of 1000 years of history across a continent with very diverse practices and sometimes distinct social norms depending on class or 'profession.' To say categorically that three meals weren't consumed is false. But it's equally false to say three meals was the norm everywhere and always, or two, or casual smaller meals with snacking.
For example, you'll see historians definitively say snacking between the established meal times was absolutely a no-no because it was considered sinful, and while that was strictly believed and followed in certain classes (or groups) in certain regions at certain points in the Middle Ages, it's absolutely NOT the case in High Medieval Southern France, where the culture definitely followed the Mediterranean model. Hospitality and food were hopelessly interlinked. If someone dropped by your house, it didn't matter what time it was, you were offering them food. Across all classes.
There are these variations across Europe today. In some parts of Europe, you absolutely are going to be served food if you visit, regardless of the time of day, whereas in other parts (particularly as you head north), you probably won't be offered food. Likewise, in some part of Europe, they eat 3 meals, in others they don't.
A lot of times historians will take published medieval sermon books and reach conclusions about the practices or beliefs of the day without taking into account that the content of sermons probably consisted of the very things they were doing rather than categorically rejecting as sinful. The sermons do inform us about the beliefs of the church. The actions they sermonize about were the things that irked clergymen, who had their own standards they wanted to impose. So, if you were being told in sermons how sinful snacking is? There's probably a lot of snacking going on and your local clergyman hates it. Otherwise, they wouldn't mention it. It's not universally the case, the clergy do have weird (almost fetishistic) obsessions that probably weren't rooted in fact, but it's a fair assumption we can learn about the realities of life from sermons with the less weird stuff.
So, while in some regions at some points in the Middle Ages, you'd expect only two meals--no breakfast, largest meal at lunchtime, in other regions the three meal system dominated. Or the biggest meal was in the evening! It depended on the work you were doing or the region. Lords would be more likely to eat breakfast than not. In fact, often lords were set up with food in the chambers so they could eat during the night. So, in that case, they broke their fast long before 'breakfast.' Which sort of plays into the theory of two sleeps.
We often don't have the kind of records we'd like for middle and peasant classes, but it's all about looking in the right places. We find interesting stuff in criminal testimonies (and inquisitorial records). We know they'd get up in the night and have sex and commit crimes together based on testimonies, and you better believe they'd want a snack after.
