AWK: trying to find instances of a value in a column; awk returning instances of Nan as well as value

I'm having a very frustrating problem using AWK and have been unable to find a solution here. It (should be!) very simple:

I have a text file 'myfile.txt' containing 3 columns. Contents are:

-101.358 80.775 3

-100.187 80.850 2

-98.019 80.976 NaN

-96.476 81.0566 NaN

All I want to do is retrieve instances where column 3 = 3. So I do:

awk '{if ($3 == 3) print $1, $2, $3}' myfile.txt

Returns:

-101.358 80.775 3

-98.019 80.976 NaN

-96.476 81.0566 NaN

For some reason it is returning NaNs as well as the 3 I'm looking for. I get the same result with:

awk '$3 == 3 {print $1, $2, $3}' myfile.txt

Returns:

-101.358 80.775 3

-98.019 80.976 NaN

-96.476 81.0566 NaN

I can overcome the problem by doing:

awk '{if ($3 == 3 && $3 != nan) print $1, $2, $3}' myfile.txt

Returns:

-101.358 80.775 3

But it is a pain to have to include this extra condition every time I use awk to find instances of some value in my files.

Please help! I'm running version 20070501.

Many thanks