I have dug a 3 foot deep trench 8 x 12 and need to place rebar in it before pouring concrete. How much rebar do i need and how do you properly place it in the hole?

Thanks for any advise

There's a good article on the proper installation methods for rebar, to be found at Taunton Press. Search category is 'Foundations & Masonry' and the sub-category is 'Concrete'. The name of the article is 'Avoiding Common Mistakes in Concrete and Masonry'. It'll cost you $3.50 for the article, but well worth the investment.

The rebar size, amount, and so forth is determined by the footer width and the load placed on the footer. A structural engineer would be the appropriate individual to consult for a specific answer to your question. If this is for a storage shed or non-habital building and you want to "ballpark" it, then a couple of #4 bars would work for a 12" wide footer. If your width goes beyond that to say 16" or 18" then you may want to put 3 #4 bars in there. Others may have differing opinions.

Question: WHY do you need to place re-bar in the footing?

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You aren't very clear on your footing, but I am assuming you have your 3 foot deep trench dug 12" wide and are going to pour concrete 8" deep in it. If you are pouring a (short) wall on this footing you may want to dowel some bars out the top of the footing to tie your wall on. Maybe bend those #3 bars for added pull out strength. You don't need large steel in the footing itself as you are not creating any substantial tension forces in this footing, it is too narrow. The steel in the footing is mostly temperature steel. You could lay a small mesh in the bottom if it is cheaper than #3 bars. If it were me and mesh isn't the cheapest then I would put 2-#3 longways then cross them with short #3 bars every 12-16" (I bet this would still be slightly overkilled). Even if you are in a heavy freeze thaw area not much steel is needed in the footing, but it will last longer if some is in it.

if he is in a heavy freeze-thaw area 3-feet is not deep enough.

Or maybe it's a three-foot deep foundation wall with no footing, for an 8 foot by 12 foot building?? Even then, unless there are going to be some colossal loads from the structure above, reinforcing bars shouldn't be really necessary, except as a precaution.

It all depends on the loads. If there ARE colossal loads from above, then a wide footing may be necessary, however, this is unlikely.