Showcase 96 #3 was released in 1996, whereas Black Canary/Oracle: Birds of Prey #1 was released in 1995; ergo, it's the first appearance.
I have always thought it was the Black Canary / Oracle book as well, but where did you get 1995 from? The wikia states it's their 1st appearance and was released June 1996.
Showcase '96 #3 was released in March of '96, but the wikia states it only have Lois Lane and Black Canary, not Oracle.
Once again, wikis are not a reliable source of information.
If you want reliable sources, try places like here or here (68 & 97 are relevant).
EDIT: Here, too.
This doesn't really tell me anything that I haven't already stated. Cover of Showcase #3 states March. Of course that doesn't mean when that's when it came out.
Also, one grading company labels Showcase as the first and CGC does not label the one-shot with any 1st appearance.