Seudah shlishit before Pesach: what's the source for omitting bread?
When Pesach starts on a Saturday night, I understand that we are forbidden to eat matzah on that Shabbat (because we don't want to diminish the experience at the seder), but that we also aren't allowed to eat bread (chametz) past mid-morning. I also understand that on Shabbat we are required to eat three proper meals (with bread). These requirements don't combine well.
I'm aware that one way to solve the problem is to have a token second meal early on and then immediately roll into the third meal, all finished before mid-morning. I attended Shabbat services once (in another city) where they paused the morning service in order to do this. (Maybe it was between shacharit and musaf? I don't remember. It was an Aish HaTorah shul.) This works, but is not very satisfying for a family experience if not everyone goes to shul. (I realize that this year few people go to shul anyway; I'm asking about the general case.)
I read an OU article that says this, unsourced:
The third meal may consist of fruit or fish or meat to the exclusion of bread, for there are some authorities who state that these foods fulfill the requirement for the third meal. These foods can be eaten after the beginning of the 10th hour of the day provided that one partakes of then in moderation.
What is the source for the opinion that we can have a Shabbat meal without any bread at all? Does this opinion apply in the general case, only when there is unspecified hardship (like a gluten intolerance), or only in the specific case of Shabbat immediately before Pesach?