Set the custom field value based on 2 drop down selectons in Jira

[udelle_govender_fnb_co_za]

Good day, 

I was wondering if anyone can help me with a code to use in behaviors. Basically I have 3 custom fields. A is a drop down, B is a drop down, and C will be a Free text field. 

When a user selects a specific option in A, then B's list will dynamically populate with a set of given options, overall based on the combination of A's and B's selection then the value in custom field C will be set to "xyz". 

I need to write up a few of these in one script by using if and else to cater for all the avail options, as each option will set custom field C to different values. 

any help will be greatly appreciated

Thank you,

Udi