How would you say the following please?

Let's say you are folding bed sheets.

Themanager says:

"Fold the bed sheets exactly in the middle/in half/in 2 so that the corners are perfectly aligned.

Now can you say

- Fold the sheet in 4/in quarters.

- Fold the sheet of paper in 3 so that it could fit the the envelop.

Thank you
'In 2/3/etc' does not work here; the rest are OK.
Students: Are you brave enough to let our tutors analyse your pronunciation?
but how would you say it than?

Fold the paper in 3 and stuff the envelop.

as most papers are folded IN 3 ????

Thank you
'In thirds', perhaps?