So after considerable research, I've decided to stay with the Fibre3 and will probably use an additional supplement - Doc's Rabbit Enhancer. It is only 14% protein, so it will be safe to add without upsetting the protein balance. It also has Vit C, which is believed to help prevent digestive upsets in rabbits and has other benefits as well.
If my local feed store (a Purina dealer) can't get it, I'll have to order it over the internet.
We'll see how it goes. I'll probably mix it with Calf Manna and feed it in the mornings with the bunns' hay. That way the protein will be more balanced throughout the day - helpful for maintaining proper gut flora.