B&C (I'm assuming you're one of them) need to be very, very sharp.
The house is a mess. Parent left it in state and A has now just piled all their stuff around Parents so it's like a jumble sale. It's depressing. I can't see A ever tidying it up and making it a proper home.
Is this just 'stuff' or is it bad enough to cause the house to depreciate in value, meaning A would have to pay less to buy out B&C?
A wants the estate to pay for ongoing maintenance of the house while probate is sorted.
As long as it is essential maintenance then that is probably reasonable. Any discussion of B&C selling to A is abstract until probate is granted. But it must be essential eg a leaking roof. Not having the roof replaced, new double glazing and a new boiler and then trying to argue that those things (which will increase the value of the house) should be disregarded in the valuation and the value taken as at the time of death.
And for the estate to pay the rates.
Council tax is paid by the occupier. A is living there so A should pay.
It sounds as though B&C suspect that A will take every opportunity to feather his/her nest at their/the estate's expense, and based on what you've said, with good reason. Remember that the will gives equal shares to B&C, and it entirely up to you what you do with your share. A cannot force you to sell to them, and you could rightly insist that the house is sold and you get your share in cash from the sale. This means that if A really wants the house to live in, rather than to profit from, you are in a good position to make sure that what you do is fair to B&C. If they are being grabby and self serving, you say no to them buying it and say you want your cash. The executor has a legal duty to act in the best interests of ALL the beneficiaries.