I imagine you could do them if your hair holds a curl well because twists are kind of like using a curling iron to give you a different curl pattern (but without the heat damage) but curly hair usually holds other curly styles well anyway. I can't give you an exact prediction on 3a hair but twists always give a round curly O corkscrew look just like braids give a wavy appearance. I don't know how to explain it, Like this:
Bigger Twists:
Smaller Twists:
I've always wondered if they did Shirley's hair in twists for that look. They probably just used a curling iron. And of course the bigger you make the twists the fatter the curls will be. And if it ends up looking too "perfect" you can always break apart the curls a little and frizz them up. Other than that twists out give some people more volume and other people less volume. On 3a hair it would give you more of a springy curl pattern, I imagine.
And for those you don't already check out
http://curlynikki.com she has a cool version of twists call the twist 'n curl. She twists and then sets the end on rollers. This gives the ends (which might turn out kind of straight) more curliness. Lots of people love it. Here's a video step by step tutorial:
http://www.curlynikki.com/2008/10/as...is-here-d.html