We were there March 11-13 of this year and the weather hovered right around 70 and varied between overcast and sunny (our last day was beautiful!!) but the weeks right before and after they had rain. However, Anaheim isn't like Northern rain..... My understanding is that it doesn't detract much from the fun!
I think March weather in Anaheim is beautiful!! It can be a little nippy in the morning, and you'll possibly need a sweater or something for the evenings, but during the day the weather is just right!!!