Home Depot has wiring chart that says I can use #2 aluminum for a 100 amp underground service I am running, that just doesn't sound right, but I bought it, please let me know. The aluminum I bought is for direct bury but only has 3 conductors. I bought a single #2 aluminum it's alumaflex (TM) AA8176 TYPE USE -2 OR RHH OR RHW-2, IT'S 60 mils 600 volts. Can I bury this cable along with the 3 conductor cable I bought? It's for the ground.

What exactly are you doing? Is this a main service or a sub-feed?

This is a 100 amp subpanel for my barn. It will be fed from a 200 amp main about 60 feet away. I am going to install a main disconnect on the subpanel along with a 100 amp breaker at the main. I was going to run the 3 wire direct bury cable I bought along with the #2 AL ground if I can. I will keep the ground and neutral separate at the panel and run a #4 solid copper ground attached to 2 ground rods separated about 7 feet.

That is fine then.
Two things, you only need a #8cu or #6al ground and you only need a #6cu to the ground rods. Although, if you have everything already then what you have is OK, it's just a bit big.

Thanks, your always a big help