3g is tiny. I think it's too small for even a cajun dwarf cray (though if not, that might be your best option!). Like ~T~ says, shrimp would be best. If you're after little things with claws, you could go for a species called Macrobrachium Lanchesteri.
I keep a couple of Lanchesteri myself, and they're genuinely quite charismatic little things.
Live for about 3 years if you take good care of them, like a balanced diet (feeding them bloodworms is fun) and grow up to about 3 inches (I think?). I'm guessing you're an Aussie, yeah?
Lanchesteri originate all around the South East asian coast, so there's bound to be a LFS that can order them in for you. (They're often known simply as "Glass Shrimp" or "Vietnamese Glass Shrimp".
Here's what they look like...
Another potential option is a Macrobrachium Assamensis (also known as the Redclaw Shrimp) -- though they grow larger than Lanchesteri, and could possibly outgrow a 3g tank. Assamensis though, have even more character than Lanchesteri. Keep them for long enough and (so I'm told) you can get them tame enough to be hand fed.
Here's a video of one.
EDIT -- Cajun Dwarf Crayfish are smaller than I thought. Only around 1.5 inches, so one (or possibly two at a stretch) of these little fellas might be ideal (provided you're not in the UK, where they're illegal). They're also known as Mexican Dwarf Crayfish, and the most popular breed is bright orange in colour.
More info on them here:
Link!
