Could Kate be Queen Elizabeth III "Catherine Elizabeth" could she change her name"?

NEVER. Kate married INTO the family - she can NEVER be a Queen.

She can call herself anything she wants. But she does not get that Roman numeral.

No. She's won't be the reigning monarch, only a consort, so she won't have a number.

No and yes.

Catherine is the wife to an heir to the throne. She will never be a reigning monarch as she is not a blood family member, a legitimate descendant of the Electress Sophia of Hanover. Catherine will be a Queen Consort, as opposed to a Queen Regnant. A consort does not have an ordinal number after her name, but the reigning monarch does-- a regnant reigns and the ordinal number identifies which similarly named monarch from the others.

There are different types of Queen:

Regnant-- the reigning, hereditary monarch. There have been two British Queens Regnant named Elizabeth, so that is why there are women known as Elizabeth I and Elizabeth II.

Consort-- the spouse of the reigning king does not have hereditary powers to reign.

Dowager or Queen Mother-- the widow of the king and , in the case of Queen Mother,

the mother of the monarch.

Could Catherine choose to use her middle name when she becomes consort? Yes, she could. It isn't likely though. She can be Queen Catherine or Queen Elizabeth as consort, but she would not use an ordinal number after her title and name.

I will always think of her as a Queen of Hearts.

Not as the Queen Consort.

She could change her name, everyone in the UK has the right to change their name, either by reputation (without legal documentation) or via deed poll (as my sister did when choosing to change her name),but as a Consort, she will not be amongst the numbered Elizabeth’s if that’s the name she chooses.

No, she has no claim to the throne, she will be william's queen consort when he is willaim V,the titles will be william V and queen catherine.

There was Queen Elizabeth I and now we have Queen Elizabeth II but, The Queen's mother was also Queen Elizabeth but, without the regnal number as Lili has already said.

No. Only monarchs take regnal numbers, and she will not be the monarch. She will be only a queen consort -- the wife of the king.

As it happens, her middle name is Elizabeth, but there's no reason for her to use it. She won't be the first Catherine who became queen consort, but again, the consorts don't use regnal numbers.