The doe probably won't miss the kits one bit - more like a vacation, probably, since they'll be coming out of the box looking for second breakfasts by that age.
However, my hesitation to show her at that point would be twofold. First, she might bring something back that the kits could be susceptible to; she'll potentially be exposed to many illnesses and parasites as she sits in the judging coop and in a room with many other rabbits. The kits' immune systems are still developing at three weeks, and I tend to not want to rock the boat, especially if it's an important litter. And yes, the travel and the show might be stressful for her too, especially since she's not an old show hand.
The other reason is that putting a lactating doe on the judging table is seen by some judges as, um, maybe you'd say a tiny bit rude or tacky? They probably won't toss the rabbit off the table, but my expectation would be that it would at least get a comment, if only a humorous one.
However, my friends and I have all shown lactating does, to make a full class so that the winning rabbit will get a G.C. leg. So, I'm not opposed to it in theory. I just factor in the risks and benefits before making the call to show or not.
