c&c3 help

You have to practice, practice, practice! Develop unique strategies with each of the races and figure out what works best for you. Build a crane as your first building along with a Surveyor/Emissary/Explorer to expand to new Tiberium fields ASAP.

I find with GDI, what you have to do is survive and tech up Mammoth Tanks with the Railgun upgrade. Everything else is just a waste of time & money. Get defenses early game, get Refineries at other Tiberium fields to enlarge your source of income, make sure you put 2 harvesters on each Refinery...etc.

Also, what you can do is build a refinery next to one or two that you already have (I usually put 2 refineries and 4 harvesters per tiberium field) and after the harvester appears, sell the refinery. This gives you back $1000, the normal price for a harvester and frees up vital space in your vehicle production queue.

With Nod, your best bet is to get Venoms + Scorpion tanks with Laser Capacitators (and Dozer Blades). These will steamroll over any tough infantry and fry the crap out of any aircraft or vehicles that stand in your way. Get 3 war factories and 2-3 airfields and get maybe 25-30 Venoms and Scorpion Tanks. Then, make sure your FORMATION ATTACK them (A + Left & Right mouse at same time + moving mouse up & down to increase width of formation). DO NOT GET AVATARS. They are a waste of time, money, and everything. They cost more than mammoth tanks but even when fully upgraded still get owned by them. Hopefully they'll patch them to make them more powerful later.

Scrin: AIRCRAFT ONLY (except maybe some Disintegrators for early game). Get your key buildings (power plants, refineries, & a nerve center) then go STRAIGHT for 3-4 airfields. Get your tech center and signal transmitter up and running ASAP and start spewing out those Stormriders, Devastator Warships, and Planetary Assault Carriers. Don't forget the Forcefield Generators upgrade at the Technology Assembler. Use the Stormriders to harrass the enemy's harvesters & aircraft, the Devastator Warships to take out AA defenses and the Planetary Assault Carriers to kill anything that moves.

Also, the Scrin is absolutely notorious for their mothership of doom.
A heavily cluttered base can be turned into a steaming pile of ash with the Buzzer-Bomb tactic. This is how you do it:

1) Summon Mothership
2) Build a few buzzers
3) Get vision inside the enemy base (use a captured GDI Command Post, or a Stormrider, or whatever)
4) Put the buzzers not directly under the blast of the mothership but nearby
5) Open up a wormhole into the enemy base near units & buildings
5) Have the mothership force-fire (Ctrl+RightClick) on the ground near the buzzers. If you did it right, it shouldn't kill them. But it should begin turning them white.
6) Immediately send them through the wormhole.
7) This is what should happen: The buzzers, after going through the wormhole, should turn white and then explode. This explosion sends out a shockwave that performs the same effect on everything within the radius - causing everything inside a cluttered base to be annihilated.

I hope these help you.
But a suggestion: Test out these tactics in skirmishes, while experimenting as well. At least you don't stain your record with losses in the skirmishes.

Cheers