trigger MyTrigger on MyObject__c (before insert) {

    Map<String, String> m = new Map<String, String>{
        '1' => 'Partner',
        '2' => 'LLC',
        '3' => 'Other'
    };

    for (MyObject__c o : Trigger.new) {
        String value = m.get(o.MyField__c);
        if (value != null) o.MyField__c = value;
    }
}