Depends on quite a few things.
1: Terrain.
Get a course profile if you can. If it's a relatively flat course like the Back Yamma Bigfoot, I'd say no problems. If it's hilly, it depends on how hilly. You may need to walk the steeper climbs.
2: Course?
is it point-to-point or around a circuit? You need to plan yoru nutrtion and how much fluid you expect to need to stay hydrated and within yoru energey reserves. It seriously NOT fun getting caught short on carbs/food and especially water.
3: Nutrition
Make sure you have enough food and water with you to complete the course. I generally consume 700ml of sports drink an hour plus a gel and maybe half a white bread jam sandwich (ie, one full slice of bread) per hour when going hard. When I am fit I do 50km (depending on terrain) in 2-3 hours, you need to have a feel for how long you think it will take you, then add another hour's supply on top in case things go wrong. If it's hot, your water needs will go up. Do ou haved a Camelbak or similar hydration pack? 1 or 2 water bottles on the bike will not be enough.
If it's on a circuit you can perhaps get famly members to hand you stuff each time you go around, depending on the length of the course you may not need ottcarry a camelbak in those circumstances.
4: Approach
Take it easy in the first half and aim to come home faster in the second half if you're looking to race. This avoids blowing up mid-course or locking up with cramps, which on a point-to-point is not a nice place to be. If not, then just enjoy the scenery

5: Have fun
This is supposed to be fun and enjoyable - relax!
6: Train
You have 3 weeks. Try and get out for some longer rides over the next two weeks without overdoing it. Say a longer ride on Sunday morning, with Tuesday and Thursday rides of an hour or so for mainenance Recover on the other days.
This is important: make your last week before the event your taper week, with your last ride before the event (I'm assuming it's a Sunday) on the Tuesday previous. A quick spin-up on the Saturday as you investigate the course is a good idea, but no more than 45min-1hour max.