For Situation 1, where the temperature of a gas sample changes at constant pressure, you would use the Combined Gas Law equation:
P₁V₁/T₁ = P₂V₂/T₂
For Situation 2, where the volume of a gas sample is kept constant while the temperature changes, you would use the Gay-Lussac's Law equation:
P₁/T₁ = P₂/T₂
For Situation 3, where the volume of a gas sample changes at constant temperature, you would use Boyle's Law equation:
P₁V₁ = P₂V₂
Note: In all the equations above, P₁, P₂ represent the initial and final pressure respectively, V₁, V₂ represent the initial and final volume respectively, and T₁, T₂ represent the initial and final temperature respectively.