All evidence points to the fact that Scarlet quite liked the Minions to begin with. Despite their unusual size & shape, they'd proven themselves (at Villain-Con) to be quite adept as criminals, something that Scarlet definitely respects. Not only did she then invite them into her home but she even went out of her way to try to make them comfortable before their big job, stealing the crown of England for her.
That all changed when Bob managed to pull the Sword from the Stone and instead of stealing the crown, something that was supposed to have been her crowning achievement as a criminal mastermind, he actually secured it legally(!!!!).
To add insult to injury, Bob then offered her the crown (again, legally) which, for someone as heavily invested in crime as Scarlet, was the most vicious betrayal imaginable.
SCARLET OVERKILL: You stole my dream! I was going to conquer England some day! There was going to be a coronation, and I was going
to be made QUEEN. Every moment was planned, I’d wear a dress so
sparkly it glowed, and EVERYONE WHO EVER DOUBTED ME would be watching,
and they would be CRYING. I was going to be the picture of elegance
and class and you PINHEADS screwed it up!
Note that her original plan wasn't to become Queen, it was to steal the crown. Becoming Queen was part of a longer-term plan which Bob achieved by accident. After that, any feelings she had for the trio went out of the window. Even an offer of the Crown was insufficient to placate her:
SCARLET OVERKILL: I want you to take this the wrong way, but I hate you. I thought I
could get over what you did, but I feel so betrayed. I think, yes, I
think we’re gonna have to break up. And it’s not you...oh wait, hold
on. It is you. It’s 100% you.
As to what her plans were for the Minions after she'd stolen the crown, it seems reasonable to assume that she would continue to to use them (as her minions) as part of her longer term strategy to become powerful enough to challenge the Queen of England.