This is probably one of the hardest challenges. Although short your jingle needs to stick somehow.
All I can say is good luck. If anyone's having a lot of trouble i can make up a fake jingle melody for you and you can write to it (I won't recheck the melody I give you to make sure there's correct syllable count, etc. if you ask for one).
EDIT: I forgot to mention, this challenge is also very practical. Anyone trying to succeed in any business needs to be versatile.
FYI, it can be longer than three lines. It shouldn't be a full length song, maybe just the length of a long verse at most. So, like, eight lines tops I'm gonna say?
And you can always do some research on how to write a jingle if you're super stuck. Remember that we don't care about little fun facts about your company/product.