What are the relationships between these 3 properties of a fluid and how would you calculate say the density when you know the viscosity and specific gravity?
Viscosity is a measure of "thickness" of the fluid. It's a resistance to shear deformation. Specific gravity is the ratio of the fluid's density over water's density. And density is just the ratio of mass to volume. To calculate density, just use the specific gravity and multiply by water's density.