all you need to now is that 1L = 1000[tex] cm^{3} [/tex]
so
Mass of 3.00L of pure water = 3000[tex] cm^{3} [/tex]*1.03g/[tex] cm^{3} [/tex] = 3090g
Mass of 3.00L of whole blood = 3000[tex] cm^{3} [/tex]*1.05g/[tex] cm^{3} [/tex] = 3150g
Mass of 3.00L of seawater = 3000[tex] cm^{3} [/tex]*1.00g/[tex] cm^{3} [/tex] = 3000g