Three Meals Ahead
Provided by Frank Lynch, who maintains a great website on Samuel Johnson
I'm fortunate to be a Johnson devotee, because Primary Source Media sells a CD-ROM that makes queries like this un piece de gateau. (It's a wonderful resource — all of Johnson, plus all the contemporary biographies about him...)
The phrases 'three meals ahead' and 'meals ahead' do not appear, apparently. Variations? (I lead with what I think is the most promising candidate.)
In Hester Piozzi's anecdotes (available in Hill's "Johnsonian Miscellanies," vol I p. 316), Piozzi writes "...of the various states and conditions of humanity, [Johnson] despised none more I think than the man who marries for a maintenance: and of a friend who made his alliance on no higher principles, he said once, 'Now has that fellow (it was a nobleman of whom we were speaking) at length obtained a certainty of three meals a day, and for that certainty, like his brother dog in the fable, he will get his neck galled for life with a collar.'"
In Rambler #12, a fictional correspondent writes to the Rambler, and at one point says, "It is strange when people of substance want a servant, how soon it is the town-talk. But they know they shall have a belly-full that live with me. Not like people at the other end of the town, we dine at one o'clock. But I never take any body without a character; what friends do you come of? I then told her that my father was a gentleman, and that we had been unfortunate.--A great misfortune, indeed, to come to me and have three meals a-day!---So your father was a gentleman, and you are a gentlewoman I suppose - such gentlewomen!"
"Meals in advance": does not appear
"Meals planned": does not appear
I've also looked at "meals", but didn't see anything additional that seemed relevant...
Summing it up, I presume a forgivable memory lapse on Pynchon's part, but that the gist is fine: Johnson considered three meals a day to be a symbol of regularity.