1. I got a hundred on today's English test.
2. I got one hundred on today's English test.
3. I got full marks on today's English test.
Are the above sentences interchangeable?
All three are fine in BrE.
They're all fine but they don't necessarily mean the same thing.
I got full marks = I got one hundred per cent.
If you just say "I got a hundred on the test", we don't know how well you did. It might have been possible to get a mark of 300, in which case you didn't do very well at all. If it was only possible to get 100, then you did very well indeed.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.
In BrE, you can also say in today's test.