When three molecules of a hexose sugar ( C_6H_ 12 O_6 ) are joined...

Three molecules of hexose sugar ($C_6H_{12}O_6$) undergo condensation to form a trisaccharide.

During the formation of a trisaccharide from three monosaccharides, two glycosidic bonds are formed.

Each glycosidic bond formation results in the elimination of one water molecule ($H_2O$). Thus, two water molecules ($2H_2O$) are eliminated in total.

Total atoms before condensation: $3 \times C_6H_{12}O_6 = C_{18}H_{36}O_{18}$

Atoms eliminated: $2 \times H_2O = H_4O_2$

Molecular formula of the trisaccharide: $C_{18}H_{36}O_{18} - H_4O_2 = C_{18}H_{32}O_{16}$

Answer: $C_{18}H_{32}O_{16}$