Also-I get what you are saying about saying it in a way it's meaningful in terms of the relationship you have with each of them. That is also true for me. The only major difference is that since we do live together there are many issues (especially threads on here) that I will send to both of them together on purpose so that they both will read it and the three of us can talk about it-because while we do not all 3 share a sexual relationship-we do all live together and raise our family together, so many day to day lives are pertinent to all three of us.
Oh yeah .. I think if I were ever in a situation where 3 (or more) of us lived together - or if I had a b/f who was single and able to be more involved in my life (which might happen in the future, who knows), I can see situations were 3 way emails would be much more appropriate and useful.