It's great that you are being proactive about your insulation. With 30-40 percent of a home's heat loss through under-insulated attics, you can save energy and dollars by ensuring yours is properly insulated while lowering your carbon footprint at the same time.
Since you currently have only 3 inches, you are wise to add more. If you can see your wood joists (rafters are on the ceiling) on the attic floor, you don't have enough insulation. The Department of Energy recently upped its attic insulation recommendation to up to an R-value of 60 which in most attics means you need a minimum of 19 inches of fiberglass batt insulation or 22 inches of blown insulation.
There are a couple options to make sure your attic is properly insulated. As you already have blown insulation, consider adding 19 inches more of blown insulation to get you up to R-60. Depending on where you live and availability, the portable Owens Corning AttiCat® Expanding Blown-In PINK Fiberglas Insulation Machine and AttiCat® Expanding Blown-In PINK Fiberglas Insulation make it easy and quick to do the job yourself. Check with your local Home Depot or Loews to see if they rent these handy machines.
If the AttiCat system is not yet available in your area, use batt insulation to add attic insulation. Given your current 3 inches, the folks at Owens Corning actually recommend you add a minimum of 16 inches of batt insulation to achieve R-60 which means you won't see your joists. When installing the attic insulation, lay temporary flooring (using planks or plywood) across your joists and work from the outer edge of the attic toward the center. Lay the new attic insulation directly over your existing blown.
Also, because you already have blown insulation in your attic, use only unfaced batt insulation with no vapor retarder. If you add more faced fiberglass insulation, any moisture that does go through the first layer may condense on the second layer which might cause water stains on the ceiling and could lead to structural damage. Rule of thumb, if you're adding to existing insulation, make it unfaced.
Finally, when working over installed insulation, always use temporary flooring such as plank or plywood to provide footing. It's fine to step on batt insulation if you must however you may need to fluff it back up if does not fully recover but it is safer and you'll feel much more stable with firmer footing underneath.
When considering using the attic for storage, first confirm the existing framing can support additional weight of framing to hold the storage and the storage itself. If you need the storage and the framing can support the weight, there are two options:
1. Leave a section of the attic under insulated and only add insulation to the areas that won't be used for storage.
2. This step is more involved and will require some carpentry but is recommended especially if you used blown in attic insulation for the project. Build up the floor area where you would like to have storage close to the attic entry way is recommended. This can be done by adding dimensional lumber perpendicular to the existing framing. Attic insulation can then be added between the new framing and plywood or osb added as a floor. This area of the attic will still be under insulated, but will be more insulated than the first option.