We should really start using hours per week instead of number of jobs. It's too easy to push an agenda with the latter. 'I work 3 jobs' reads differently than 'I work 45 hours a week'
Number of hours can also be misleading. I work 40 hours a week at one job. Someone who worked 40 hours a week at three jobs would certainly have more difficulties at work, like more commutes, more context switching, not having the benefits that arise from a full time job, etc.
It does matter how many jobs you work though, because low-wage employees arrange things so nobody gets 30 hours a week to qualify for full time benefits. Also the kinds of jobs that people who work 3-4 jobs often work typically have chaotic schedules that aren't set well in advance, meaning that they inevitably come into conflict creating stress and the potential to be fired for not being able to show up at one of them.
Realize I meant to type "employers" here.
The issue is each job might have you on call.
So you can have some 80 hour work weeks and 8 hour weeks.