I have an existing 100/125 amp meter panel with 4 used spaces (2 100amp double breakers = 4 spaces used). The power comes in from underground through metal conduit into the bottom of the panel. Power goes out the back to 2 basement sub panels. I am looking to replace this panel with a new 125 amp meter panel with 4/6 spaces. I am wanting to run a third sub box with power going out the bottom through 1-1/2, or larger, to a shed 100 amp sub box max 90 feet away. I am looking to use SER 2/0, 2/0, 2/0, 1 AL cable. Both ends of the PVC conduit will be secured to the meter and sub boxes. Also are the 2/0's for Hot, Neutral and Common and the 1 is the ground? Where will I attach the ground?
Since your description mentions that you have conduits connecting the boxes, you should not use cables at all. They have terrible conduit fill (because they have to be treated as a round of the largest dimension for oval/flat cables) and they are quite difficult to pull as compared to individual wires.
It's concerning that you are proposing DIY service entrance wiring and yet "Common" is one of the wires you think you have. In USA/Canada spit phase, you have Hot, Hot, Neutral and Ground. There's 120 volts from hot to neutral (in both cases) and 240V (or sometimes 208V if from a 3-phase transformer) between the two hots. The Ground conductor can be smaller than the hots and neutral.
The ground conductor attaches to the grounding bar in the sub-panel, and to either a grounding bar or to the neutral bar in the main panel, but in your case the bond (and main panel) is probably at the meter-main, so all other panels are sub-panels and should have grounds on grounding bars only, and neutrals isolated from ground, as neutral and ground are only to be bonded one place, at the service entrance. The shed will also require grounding electrodes - if it's not built yet, put a concrete encased electrode in the foundation (they cost almost nothing if planned for, and are the best grounding electrode you can get.) Otherwise a pair of rods at least (but more is better) 6 feet apart is the minimum.
For 90 feet at an honest 100A, 1 AWG aluminum is quite sufficient, and 2/0 is massive overkill. By which I mean a waste of perfectly good money on a wire size that's not going to save you anything, unless you have unstated plans to run a lot more current out there. For considerably less money than that, if you can manage with a 90A feed breaker to your 100A sub-panel, (or 150, 200A - the sub-panel only needs to be equal to or greater than the feed breaker, they don't have to match) 2AWG aluminum is sufficient, and tends to live in a price sweet spot.