"How useful would the chair with three legs be?"

Translation: Jak užitečná by byla ta židle se třemi nohami?

[KTucholsky]

Why not "ta zidle by byla" but "by byla ta zidle"?

[Shathu_Entayla]

Why is "tremi" instead of "trema", here? Isn't it dual anymore?

[relox84]

Well dual, as its name suggests, is used when there are two of the noun. In standard Czech, the forms in -ma are only used when they are exactly two of the noun, and -mi is used when there are more than two.
In colloquial Czech however, it's not uncommon to use -ma even when there are more than two.

[endless_sleeper]

Not exactly.

The main point of confusion here is that dual is used with body parts (kočka se čtyřma nohama) but not with inanimate things (stůl se čtyřmi nohami)

[Shathu_Entayla]

DAAAAAAAMN! Ok! Gotcha! Dekuju moc! Give ya lingot!