Dextrose is a hypotonic solution as the dextrose is metabolised, leaving only free water. Only 7% of the fluid stays in the intravascular component.
0.9% saline equilibrates in the intravascular and interstitial spaces, with 25% staying in the intravascular space. It is useful in resuscitation, but using it as the sole maintenance fluid can result in hyperchloremic acidosis.
Hartmann’s is the most physiological fluid out of the three as it contains more electrolytes. A common misconception is that Hartmann’s causes lactic acidosis, but Hartmann’s solution contains lactate (the conjugate base), thus is actually an alkalising solution.