It took me a long time to work this out and when I did, I felt like putting it on CheatsGuru Community! This is a secret to tell you which dogs are good for which contest. Lets get on with it.
Disc Competition:
A challenging contest, but you will still be able to conqueror it with patience. I would recommend you big dogs. You can also use a dog size between middle-big because they will do a lot of jumping. I recommend:
Labrador Retriever, Shih Tzu, Shetland Sheepdog, Miniature Schnauzer, Golden Retriever, Boxer, German Shepherd or Siberian Husky. Everyday before you enter a contest go to the park and play fetch with your tennis ball. Throw it as hard and far as you possibly can. This is a good warm up to get you dog's speed increased. Now to the main subject. If your dog is newly purchased, give it a disc which has different color to all of your other toys. If it is old, use the disc you last practiced with. Then throw the disc out weakly if its new, then when it gets used to it throw it a little farther. Increase the distance every time. Soon you'll be throwing it really far! Whenever your dog brings it back to you, pat it until it sparkles. Practice until it doesnt want to play anymore and you've succeeded! I think you already know this but, only practice in empty parks. After the contest, go to the park again. Of course, if your dog is on Expert Clas.s+, dont bother patting it in contests.
Agility Trials:
Pretty simple. I would suggest you perfectly middle dogs like:
Shiba Inu, Pug, Yorkshine Terrier, Cavalier, Miniature Schnauzer, Miniature Pinscher and Beagles.
Before contest go to the gym at least twice a day and focus on one hurdle and jump over it 9 times without misses. Then go to the double hurdles, poles, see-saws, tunnels and do the same thing. After that do the whole obstical course until a message comes up that says:
(E.G.) has had enough play today. Maybe it's time to get on with your walk.
Then of course go to the contest and be amazed by your dog!
Obedience Trials:
Five words: Hard, hard, hard and hard. Any dog will be perfect because it doesn't include physic. Check the rules of this contest before practicing. Use two days to learn the basic commands. Day one learn: Sit, Lie Down and Spin. Day two learn: Shake, Jump, Beg and Roll Over. Using day three I'll teach you some commands to really impress the judges. One: Stand Up.
Get your dog to beg and lift up its paw. Then quickly touch the training icon then say: stand up.
Two: Ballet.
You can practically call this whatever you want but I called it ballet because it spins around like what ballet dancers do. Get your dog to beg then say: Spin.

Good Luck with these contests!