You have to think carefully about this and make your own decision. What's more important to you, having this particular job or completing the process of becoming Catholic?
Is there any particular reason you have to be at a specific Mass for the next 3 Sundays? Can you attend a vigil Mass the night before if you're scheduled for a Sunday morning?
If your church/priest won't budge, and you're offered the job on the contingency that you must work those Sundays, you will have to choose - either forgo RCIA for now, or forgo the job. If you forgo the job, you can explain that you would be available on Sundays after Easter and that you'd like to be reconsidered later on.
I hate to be so blunt, but as I see it, those are your options.
Good luck with whatever you decide.
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