A 3-gallon size may not give you instant gratification, but is probably an ideal size for easy planting and best establishment. Sounds like you have the right size!

3' spacing sounds really tight! These mature to around 20' x 15'. I think I would space them 8'-10' apart, even if I wanted them to fill in quickly.

I'm not sure what you mean by "constant water". These are not aquatic plants. Maybe you meant that you'd try to maintain soil moisture at root level, which would be ideal. Check for soil moisture, with a finger stuck down a couple of inches in the soil, to determine soil moisture. You should only water when needed (when the top of the soil starts to dry out but the soil down at root level is still moist).

Trimming new plants does not promote new growth. Put your pruners away unless there are broken, dead, diseased, or rubbing branches. Trimming them would be robing them of energy and redirecting energy into covering over wounds rather than growing.

Here is a link that might be useful: Planting a Tree or Shrub

Thanks for all the info! I always thought the closer you planted them the better because they would grow in faster, but I guess in this case seeing as how big they get 8-10' will be good and I will be able to puchase less now, so that works out.
What I mean by constant water was pretty much what you hit on, i figured since they were still so small to constantly water them at the root level. I was thinking about running a soaker hose the length of the bushes to get water right to the roots. Do you think this is a good idea? If so how often would you say I should turn that soaker on for?
I will stay clear of trimming them for quite some time then, thanks for your help and that article helped a lot too! I appreciate it.

A soaker hose would work OK and keep you from having to move the hose so many times. I prefer drip irrigation for this, though. Soaker hoses usually have a fairly short lifetime (almost ridiculously short in some cases). Drip irrigation doesn't cost much more (especially if you don't count the filter or pressure reducer, which don't have to be all that expensive themselves) and can last a much longer time. Drip irrigation can also be much more targeted (more efficient).

It's not really a good idea to specify a frequency or duration for watering without being familiar with the specifics involved and the environment/soil conditions. What might produce near drought conditions at one site might produce a swap at a different site. Check conditions by checking for soil moisture down at least a couple of inches in the soil. Fast draining soil will require more water and more frequent water than moisture retentive soil. Temperature, rainfall, and other environmental factors will also affect how often you'll need to water. Less frequent, deeper watering is better than frequent, brief watering, but required times and amounts of water will depend on conditions.

Planting too close to form a hedge or privacy screen will typically result in some of the plants being shaded out or damaged by more rapidly growing neighbors. If an immediate effect is desired, you could plant closer but with the idea in mind that down the road a season or two, some of the plants will need to be removed for the maximum health and best appearance of the hedge/screen. I think Brandon's recommendation for spacing is just about ideal.

Soaker hoses are an excellent method of delivering water to where the plants most need it - right at the root zone. How long and how frequently you need to run the hose will depend heavily on your soil type. Looser, freely-draining sandier soils will require more frequent watering than heavier, clay-based soils. Weather will also play a factor - hotter, drier climates will need more frequent irrigation for new plants than milder or damper ones. The best suggestion is to run the hoses for a period of time then measure how deeply the water penetrates. The emission rate of soakers is pretty low (or should be) - generally 1GPH per foot of hose or less, so it may take upto several hours for sufficient water to be delivered. Ideally, it should penetrate at least 8" or to the depth of the root ball. Same with frequency - you will need to check periodically to see how dry or moist the soil might be. Mulching well can help to conserve soil moisture and often reduce the need for more frequent watering.