50% is pretty darn good for an L3. sounds like you're on the right path. just keep doing matches and you'll get more and more each time. but I personally will use their position or description if needed. like in a heated pick and go sequence near the line...I need to push a pillar and I see he's a prop I may say, "PROP last foot" or "big man - back up." point right at him with one hand and make eye contact if possible...he'll jump.
at the tackle - nothing wrong with, "black tackler roll" or "red tackler out."
in time you'll pick up more and more as the game "slows down" for you.
Look at the back row & SH's. remember what they look like and their numbers, as it'll be them you speak too most. the rest are normally on the floor, so you should be able to catch the number, if you can't, try moving so you can.
Agree with Robbie. Use your warm-up time to familiarise yourself with both 7's in particular: hair colour, tattoos, general build - those are the guys you'll need your eye on the most. Once you've got them, do the same for the 6's, 8's and 9's in that order. That'll increase your percentage dramatically.