In addition to all the good points made, my biggest concern wouldn't be having it done in three weeks, but not knowing if it was any good before entering.
Yep. This is a cart before the horse kinda thing. Make a good beer, THEN decide to enter it. Don't decide to enter, then hope you have a good beer.
The problem is, this is Belgium & The Netherlands, not the US of A. There are about 3 competitions per year in the 2 countries combined. There is almost no homebrewing culture here. I can't brew a beer and sit on the porch waiting until a competition calls my name.
This aspect of the debate points to what I have been trying to say about competitions.
The reason home brew competitions exist is to raise money. If home brew competition organizers organized competitions simply because they want home brewers to be rewarded for great beer, the competitions would be free. So only submitting beer that you know will win, or should win, is not the desire of the organizer. If that were so, in a perfect world, they would only get one entry per award given. Not a good fund raising plan. They want as many beers as they can handle. So I say, enter whatever you want.
Having said that, if you want to do well you might consider entering your best beer. Or if its a popular competition, like NHC, and you are only allowed to enter X amount of beers, enter the best of the bunch.
But what you are doing is perfectly fine. In fact, you are ahead of the curve because you are actively seeking suggestions for how to do the best job in the short amount of time.
To summarize, if anyone is considering entering a competition, I say go for it. Just realize the reasons to enter. 1. Help a local club or brewing promoter raise funds. 2. Its just for fun. 3. Dont expect awesome feedback. In fact, the score sheets are likely going to disappoint you if your expectations are high. If the judges dont like your beer, just remember... they are wrong! ;-) 4. If you end up with a ribbon or medal, ride the glory! You are clearly an awesome brewer. ;-)