Question

in an essay do i write the letter 3 or three?

Answer 1

Always in letters unless it is larger than 100.

Answer 2

Limobean is mistaken.
Strictly speaking in whole numbers, you should write out any number that is a double digit. So, any number over 9 should be written out. Also, any number that begins a sentence, regardless of the number of digits, should be written out.

Answer 3

Use numbers (numerals) in these instances:

1. For all numbers 10 and above: "There were 17 students in class."
2. All numbers below 10 that are grouped in comparison to numbers 10 and above: "Ony 5 of 17 students passed the course."
3. When using numbers immediately before a unit of measure: "a 5-minute wait"
4. Numbers that represent statistical or mathematical functions or formulas: "a ratio of 12:1"
5. Numbers that represent time, dates, ages, sizes, scores, money, and points on a scale: "It happened 5 years ago"; "a roomful of 6-year-olds"; "$40."
6. Numbers that represent a place in a series: "week 7 of an 8-week diet"
7. In a list of four or more numbers: "We had 1, 2, 5, and 8 pieces, respectively"