Couple quick easy things you can do to make a 3 look better. Go faster and pop harder. Seriously, most people early on are timid about going too fast or going too big. If you go faster and bigger, you'll have more time and won't have to rush the rotation, which almost always makes a trick look hucked and penciled.
On that note, even if you're not yet comfortable enough to throw a grab in there, suck your knees up and really engage your core like you're about to be punched in the gut. If your body is all over the place, legs straight down and a little splayed you'll look sloppy.
Pop before you start to rotate. Sounds obvious, but almost everyone will not pop enough because they're too occupied with spinning. If you're finding you have to begin rotating immediately or having to generate momentum with your arms and shoulders before you pop, you're probably not hitting a big enough jump or taking enough speed into it.
If you can get a few 3's on film, it'll help immensely. Seriously, I'm not saying make a shitty edit and spam it, but get some shots solely for trying to see what you're doing. Where's your head? Your arms?
Throw them everywhere and all the time. Same rule applies to 3's as any other trick you want to improve....do it constantly. Do it casually on side hits or off of moguls. The more familiar with your air sense, where you are in the trick, you'll start to unconsciously make small corrections.
Add grabs! They make everything look better (though shifties are tiiiiight). I'd suggest critical, safety, mute, and japan. They all allow you to stay in a compact position and don't mess with your axis nor do they cause you to open up or "reach" against the rotation.
