What does within 3 days mean in this context?

Im in the proces of applaying for a job, and this Friday (31st of June) afternoon I got an email from the recruitment team saying thy vewed my application and think I'm fit for the role but they need to test me so they gave me a test assignment that I have to submit within 3 days...
i noticed and read the email on Saturday the first of augst (basically the next day) and I wrote them back to thank them for the opportunity and say il get back with the asigment. On Sunday I couldn't get it done because of personal issues, so I send it today, Monday 3rd of August. At around 6 pm. Now I'm a bit confused, if the 3 days dead line was including the day they sent me the email, and I was suppose to deliver it on Sunday or i calculated right and Monday is still in the green. Maybe its silly as probably no one would read the mail on Sunday anyway... i just worry it might be too late... what do you think?
What does within 3 days mean in this context?
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