If you're on a ratio of 1:20 and you are having a meal with considerably less than 20gs of carbs than yes, is fine to have no insulin with that meal. If you have your meal uncovered I would be carful to do a post prandial test at 2 and 3 or 4 hours after eating just to see what effect it has on your blood sugar and to check to see if you need to consider bolusing for protein - if you eat a meal without carbs your body can perform gluconogenesis turning the protein into glucose to raise blood sugar.
You don't have to take 3 shots of fast acting a day. That's kinda the point of DAFNE - just take as much as you need to cover whatever you choose to eat. If you were having carby breakfast lunch and dinner along with elevenses and afternoon tea you'd do 5 shots, if you skip lunch you don't do a shot for it. If you have a snack or meal that doesn't need a shot, just don't do a shot.
You should ask your DSN for a half unit pen on a 1:20 ratio it might come in handy.