Will I be at a disadvantage? (very few extracurriculars due to circumstances)

Your three example schools have different views on the worth of working versus other extracurriculars.

At MIT and Stanford, work experience is “considered”, but (other) extracurriculars are “important” (MIT) or “very important” (Stanford):

http://www.collegedata.com/cs/data/college/college_pg02_tmpl.jhtml?schoolId=186
http://www.collegedata.com/cs/data/college/college_pg02_tmpl.jhtml?schoolId=781

At UCLA, work experience and (other) extracurriculars are both “important”:

http://www.collegedata.com/cs/data/college/college_pg02_tmpl.jhtml?schoolId=1093

UCLA and other UCs do emphasize opportunity for California residents from low SES backgrounds, so it is not surprising that their admissions processes and criteria are designed to maximize the ability of such students to apply and be admitted. The result is a significantly higher percentage of students on Pell grants than at many other very selective schools. Note: a financially needy non-California resident will not be able to afford to attend UCLA or other UCs, unless s/he earns a rare top-level merit scholarship.

In contrast, many other very selective schools have admissions processes and criteria that favor applicants from high SES backgrounds (without needing to directly considering financial need), resulting in entering classes where half are non-financial-aid students (i.e. family income > ~$200,000 for high list price private schools with good financial aid), and a much smaller percentage are on Pell grants. This is presumably by design, to keep within the financial aid budget.