I don't think you can use any kind of spacers other than the default metal ones that come with A3000. The motherboard has no holes for them. So basically, you need to secure your accelerator card by using a pair of non-conductive washers and a pair of screws. You can only do this for the left and right hole on the side that's close to the cpu slot connector. The other two holes on the other, far side are useless with A3000.
I like this method a little better since once your accelerator card is screwed in, in your A3000, it can never pop-out as it can with A4000, during movment or even shipping of your case. :-)