nested IF function

Here, try this:

=LOOKUP(D3,{0,19,28,36},{"A","B","C","D"})

(wich will result you with error if you enter negative number)

or this

=LOOKUP(MAX(0,D3),{0,19,28,36},{"A","B","C","D"})

(wich will return A even for negative numbers)

From an embedded IF perspective....

An IF will exit upon first TRUE result so you need not repeatedly use prior tests.

Also we're assuming the value being tested does not reside in the cell you're placing the formula - your post is unclear.

=IF(D3<=18,"A",IF(D3<=27,"B",IF(D3<=36,"C","D")))

You might want to validate D3 is a number as well...