Make an original game using an asset pack as the base. 3 weeks is a short time to make all original assets, but if you use an asset pack or two as a base, it's easier to make original work.

The rules are simple. Your game must be completely different in terms of gameplay from the base. So for example if you use a shooter base, you could turn it into an RTS.

I used this concept for my own game and I realised it would be fun to share this with others.

you can use any asset pack you like but here is a link to some on the unity marketplace.