DD is just turned 3 and is pretty good at saying please and thank you, so on that basis I would say it's not to much to expect. If she says 'I want' I say she'll have to ask for it nicely, then she turns on the fluttering eyelashes 'please Mummy please can I have a biscuit please'!
She doesn't get what she wants if she doesn't ask nicely.
She's also quite stubborn, but we tend to do better if we acknowledge how she's feeling, even if it is unreasonable (of her). E.g. 'I know you're cross with Mummy, are you cross because of x?' Then a bit of sympathy, 'yes I know sometimes I feel cross too...' etc. And we then usually get what we need by playing on the 'are you a big girl, because big girls are really good at y', etc.
Or can you turn it into a game so she will end up laughing and break the cycle of stubbornness, like asking her to say 'please' in a really funny pleading way 'oh pleeeeeeeaaaaaase DD say please because I really want you to have this biscuit, pleeeeeeeeease' etc. Or maybe there's a please & thank you song somewhere or you could make one up.
Also, and I'm sure the answer to this is yes, but do you say please and thank you to her directly? Not just to others in the house? E.g. make sure if she gives you her empty cup/plate or gives you a cuddle or if you want her to do something, that you also say thank you/please etc. as relevant. Sure you already do this, but sometimes people are going so mad modelling good behaviour in front of the DC, they forget to use it with the DC themselves!
D