Regular expression HELP ;)

Hi

I m far from a regular expression guru and im stuch with a relativly simple regular expression to work out.
This is my try at getting it to work : [0-9a-zA-Z\.\ _\ -]{3}
What i want is an expression that can contain a-z in upper and lower case, 0-9 and . - _ but only 1 of each.  o and the whole expression must be at least 3 characters long.
 
Please can someone give me a hand, its 330am and im tired and desperate ;)
Shaun

...