Politeness of a follow-up E-Mail regarding administrative task
Rather than just asking if they received the previous email, reiterate what it is you're asking about, why you're asking them specifically, and include a deadline. This saves the recipient from having to trawl back through three weeks' worth of email to find your original request. So, you might want to send something like

"Dear Professor X,

Is it possible for you to fill in this form that I need for [insert reason here]? I'm asking you because [insert reason here]. The [admin people/ university/ department] need me to send it back to them by [deadline]. I've attached the form to this email. 

Thanks in advance for your help!

AnotherStudent"