I'm guessing it is a silver aro? Your absolutely correct that a varied diet is best, I believe this with any fish, but especially true for aros. OK, my silver would not touch things such as shrimp or food sticks etc. until it was around 7-8 inches, until then I fed him a mix of bloodworms, crickets crickets crickets, and it would absolutely pelt live minnows. NEVER GOLDFISH!!!!! Don't worry your 3" will be 7 to 8 " QUICK!!! These guys grow so fast! When mine got to around 6 inches I would (for lack of a better word) starve/fast my aro for a day or so then introduce floating sticks. At first it would grab one, then spit it out, over and over again, but once it reached about 7-8" it seemed to become a eating machine overnight. It now is around 22" and eats anything!!! My mix is beefheart, shrimp, small bluegill that I "de-fin", hikari sticks, and an occaisonal lizard that I catch! It eats any and everything. Have patience, keep trying and you'll see that your aro will eat anything.
Good Luck, Any pics???