Previous extra section ratio implies the reverse of what is true/doesnt really capture the concept. The M+2 peak (37Cl) is smaller than the M peak. so the M+2:M ratio is 1:3 not 3:1. Moreover it's not that chlorine or bromine has a ratio to the M peak. Both the M and M+2 peaks contain chlorine or bromine just different isotopes.

Explanation of Mass Spec including relative abundance of chlorine and bromine.
https://www.khanacademy.org/science/ap-chemistry-beta/x2eef969c74e0d802:atomic-structure-and-properties/x2eef969c74e0d802:mass-spectrometry-of-elements/a/isotopes-and-mass-spectrometry

Another explanation of mass spec (not directly relevant to Cl and Br but provides further general evidence for suggestion)
https://www.khanacademy.org/test-prep/mcat/physical-processes/atomic-nucleus/v/mass-spectrometer

Not Primary MCAT Source but directly discusses this topic for further support.

https://chem.libretexts.org/Courses/Providence_College/CHM_331_Advanced_Analytical_Chemistry_1/16%3A_Molecular_Mass_Spectrometry/16.09%3A_Organic_Compounds_Containing_Halogen_Atoms