Try using the Switch function.
Here is an example
http://www.bidn.com/blogs/ChrisAlbrektson/bidn-blog/1478/ssrs-switch-function-examples[
^]
Edit:
If using Switch function is not an option, you can use nested If as shown below.
IF(Parameter=2,IF((a/b)-1)>3,"red",IF((a/b)-1)>(-3),"Green","")),
IF(parameter=3,IF((a/b)-1)>0,"red",IF((a/b)-1)<0,"green","")),"")
)