〜ならまだしも
X would be acceptable, but
Noun / Plain form + ならまだしも
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ならまだしも grants that a milder case could be tolerated, then complains about the actual, worse case: 一度ならまだしも、三度も遅刻するなんて, "once would be forgivable, but being late three times?".
まだしも means "still better, more acceptable". The first half is the lesser evil, and the second half is what really happened, often ending in なんて, とは or a criticism.
It's very close to ならいざ知らず, and the two are often interchangeable. ならまだしも is a little more conversational and focuses on "tolerable vs. intolerable", while ならいざ知らず suggests "I can't judge that case, but this one…". ならともかく is the most everyday of the three.
Examples
一度ならまだしも、三度も遅刻するなんて。
Once would be forgivable, but being late three times?
冗談ならまだしも、本気で言っているから困る。
If it were a joke, fine, but the trouble is he means it.
少しならまだしも、こんなに高いとは思わなかった。
A little expensive would be fine, but I never thought it would cost this much.
自分が損をするのならまだしも、人に迷惑をかけるのは許せない。
Losing out yourself is one thing, but causing trouble for others is unforgivable.
雨ならまだしも、雪の中を歩いて帰るのは大変だ。
Rain would be one thing, but walking home in the snow is hard.