She LOVES these books and is already asking for 4A and 4B. But I think that maybe I would like to go a different direction? DH thinks she is not "really" learning 3rd grade math and that she will be fine and I shouldn't worry. (Boredom was an issue for her this year.)
Suggestions for things that would keep her math happy but not necessarily covering the curriculum she will see next year?
First, I bet she *is* really learning 3rd grade maths! As I see it you have two choices: let her get so far ahead that the school will see that they have no choice but to do something special with her; or switch now to something off the mainstream. Frankly I think given how far you've gone on the first path already, and especially since she's begging for more, I'd go further along it - she's probably already learned a substantial amount of what the GT programme were going to teach her next year! So let her do 4A and 4B, make sure she does them thoroughly, make up problems on the earlier stuff for her and check she's really got it; go on higher if and when she needs it too; and then explain the situation to the school at the start of next term and let them deal with it, it's their job after all!
If you do want to go sideways, I think there were good ideas in your earlier thread on this - I'd suggest the Murderous Maths books especially as being good for recreational stuff. Won't meet your DD's workbook desires, though, which is one reason for suggesting that you just go with what she wants, provided she's really learning the stuff properly.
