3 weeks isn't very much time, so just keep your goals realistic.
As far as what you can use for trail obstacles .... use your imagination!
I use old wooden fence posts to make parrallel lines or "L"shaped lines to practice backing through and sidepassing through.
Have a gate anywhere on your property? Use that to practice opening and closing a gate horseback.
Use a blue tarp or a large piece of cardboard to simulate going over a bridge. You can even "decorate" the sides with branches or random objects you have sitting around the yard (bike, buckets, flower pot, lawn mower, etc).
Practice opening and closing your mailbox from your horse.
Again, use your imagination!!!
Also, most of what you do in a trail course simply has to do with BODY CONTROL. You should be able to move the hindquarters with your leg, move the shoulders with your leg, and sidepass. Your horse should walk, trot, and lope nicely on a loose rein. You should be able to go from a standsill, straight into a lope on the correct lead (with lots of practice). Etc, etc. If you can precisely control all of your horse, then you should be able to move them through any obstacle course.
∞•*˚ Βгįťţαňγ ˚*•∞
It is not enough to know how to ride; one must know how to fall.