"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.