Where on God's green earth is Dad in all this, and why isn't he doing these things for his own daughter?!

If you would like, you can try making smoothies with fruit/vegetables depending on the combo I add a little sugar. That's how I can get my daughter to eat vegetables sometimes. You can try a fruit/cucumber/ little vegetable like spinach or something.

I understand what you are saying about reinforcing learning- if you want to continue trying I think every little bit helps. My 3 year-old can't tell you how old she is or trace her name well, but can do the other things you listed and some other things.

My 3 y/o is currently getting on my nerves, but with the pandemic, working from home, and no one else to watch her I'm thinking this is normal. I know that she has a lot of energy and just isn't able to use it all. On the days we can go outside to the beach, park, backyard, whereever she is 'better/more behaved/exhausted'. Yesterday, as we were driving out of the neighborhood she waved at some kids playing on the sidewalk.

It's kind of you to care, but really you are not responsible for how SD turns out.  Nothing you do will make much difference.  You're trying too hard, and you care more than the parents do.

She has a BF and BM. It's there jobs to teach her. Not yours.  Let them handle her. You have your own child to worry about. 
You should never do more then the birth parents.  That includes babysitting there child.  Your future SD does not want to be a happy family with you.  

I agree with prior posters that your expectations of a 3 year old may be a little high. My SD6 couldn't do a lot of what you described at 3-1/2. She still successfully finished kindergarten this week and is doing fine. When she started 4K, many of her classmates didn't know the alphabet, couldn't count very high and didn't write their names well (or at all). They learn so much in preschool. It's nice if you have a parent who reads to you and works with you on letters, numbers, colors, etc., but kids succeed even if they don't have that. With an October birthday, this child is more than a full year away from 4K. She'll be fine even if your DH and BM don't teach her much of anything.

I also wouldn't assume that BM is telling her to be different or defiant with you. 3 year olds can be a PITA.My skid delayed her terrible twos until she was three! She also would have been happy to eat nothing but chicken nuggets, spaghetti and pizza.  Where's her dad in all this? Can you let him handle her?

Ahhh...The "Saviour Stepmom" I know, because I was that Stepmom too. I went out of my way to make up for all of Skids BM's shortcomings. Planned amazing weekends, helped with school projects, purchased all summer reading materials, took them clothes shopping for everything they needed because the mother never spent a cent of the CS on them. It was all for nothing. I haven't seen or heard from SD in almost 7 years. Save all that love and attention for your own baby. It's not your problem. And I know that sounds harsh as all hell, but believe me, that kid will turn on you and side with Mom the first chance it gets. Save yourself the heartache.

Same as above posts. Toddlers are notorious for a reason. Occasionally one will be more easy natured. If your baby is easier count your blessings and enjoy. It's luck of the draw most of the time. A consistent environment may give your child a bit of boost over SD also. But I wouldn't worry about it or compare them. At that young age kids abilities can vary greatly. I know some 5 year olds who can read very well and others who have to work hard to write their name. Both have excellent parents who love them to pieces. They will likely both be fine. Also at 3/4 it's usually not yet apparent if they have a learning or development problem. Developmental milestones become more consistent and pronounced as kids get older. I'd be more concerned down the road.