"How useful would the chair with three legs be?"
Translation:Jak užitečná by byla ta židle se třemi nohami?
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.
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)
Second position rule for "by".
And also different meaning if you tried "Jak užitečná by ta židle byla se třemi nohami?". That is "How useful would that chair be with three legs?" As if you were about to cut one leg from a four-legged chair.