I don't see it as "back talk" or "rudeness" but as an "expression of their independance"
I still don't like it. I feel like at that age, they are still so young. They have no way to verbalise their displeasure tactfully and so their opposition comes off as rude and backtalk.
For me the key was to first recognise that if I want my children to be able to express themselves freely(going against the children must be seen and not heard opinion here) then they will go through a stage where the freedom of expression may seem terribly offensive at some point.
So I take it upon myself to model tactful disagreement such as in interaction with them, their dad, my parents, inlaws, teachers etc.
I also spend lots of time talking about how we say this "kindly" and not "rudely". outside of when the actual disagreement occurs.
My older children are 14 and 18 and they are pretty good. 14 year old occasionally yells at me, 18 year old is out of the house and pretty tactful and not rude anymore..yeah it takes that long
My 5 year old is a very sensible girl. When she has a moment of "rudeness" I gently remind her "we've talked about this, you may disagree with me respectfully, you may say no respectfully"
My almost 3 year old(end of October)..she just yells at me at this point
She's very verbal and I'm sure some would consider this backtalk and rudeness but I just consider it a phase and I don't really sweat it. She loves me and wants to be nice, she just doesn't know how to yet. It will come.
Also, I had to set aside my "hurt" when they said stuff like "mean mommy" because me saying no to something like chocolate icecream for breakfast, particularly if we didn't have any" wasn't mean but to them, that's how they felt I was being towards them.
After 4 kids, one thing I realise..most things pass. Most things really are a phase. I generally have a "help them through it" rather than a "nip it in the bud" attitude towards these things because I've learned that shortens the phase generally.