require entry in field if certain value selected from another

[mikkistr]

I have one field on a form with a drop down list and choices are "1", "2", "3". If 3 is picked then I want to force the user to put in a value in a different field on the form, without always forcing it. The user must be able to leave it blank at times. Also, if 1 or 2 is picked then I want the different field to only allow certain values depending on what is selected in the first one.

Thanks ahead for any help!