Oliver (Christchurch Cathedral School, UK) has shown his solution clearly in a diagram.

Plan view	Side View

Josh (Ampthill, UK) has used words to give the same solution:

Cut in quarters vertically at the top at right angles and then cut horizontally.

Sam (Nanyang Primary School, Singapore) came up with a different approach:

I think that the solution is:

1. You cut the doughnut into halves.
2. Then you cut it into quarters.
3. Put all the 4 pieces into a stack and cut into halves.
4. You will get 8 equal pieces using 3 cuts of a knife.