Three tier Validation Rule / Validation chain?

Question

I am stumped so I hope I can present my question in a manner that allows me to get some support. I am creating a Lead validation rule that in essence is 3 tiers i.e. if tier 1 is selected require that tier 2 is selected and if this value in tier 2 is selected require tier 3 text to be completed. Here are my picklist requirements

Lead Source           Lead Source Details                       Complete Text Field

Wholesaler
                          >ABC Wholesaler
                          >XYZ Wholesaler
                          >Other if other is selected complete   [text field]   

Personal Referral
                         nothing else here

Trade Show/Event
                         nothing else here but we need to >   [text field]

TBP Event
                         nothing else here but we need to >   [text field]

Website
                         nothing else here

Other
                          nothing else here but we need to >   [text field]

When this was only a 2 tier picklist I used this validation rule but now I can't figure out how to rewrite for 3 tier

ISBLANK(Lead_Source_Details__c) && ( ISPICKVAL(LeadSource, "Distribution Partner") || ISPICKVAL(LeadSource, "Event/Trade Show") || ISPICKVAL(LeadSource, "TBP Event") || ISPICKVAL(LeadSource, "Other") )

Answer 1

This would be a cleaner way to see what is going on and easy to extend

AND('1'=
     CASE(LeadSource,  /* set Case result to '1' if text field is required */
          'Wholesaler',CASE(Lead_Source_Details__c,
                                        'Other','1',
                                        '0'),
          'Personal Referrer','0',
          'Trade Show/Event','1',
          'TBP Event','1',
          'Website','0',
          'Other','0',
         '0') ,
     ISBLANK(TextField__c)
)

Answer 2

A validation rule's formula should be true when rules fails, so we need to have criteria for each failure condition, combined with an OR ( "||" ). If I read your table correctly, here's how to express your requirements. I used inline comments to highlight each LeadSource's section of the formula.

/* first check Wholesaler */
( ISPICKVAL(LeadSource, 'Wholesaler')
  && ( ISBLANK(Lead_source_Details__c)
       || (ISPICKVAL(Lead_Source_Details__c, 'Other') && ISBLANK(TextField__c))
     )
) ||
/* Personal Referral has no other requirements */
/* check Trade Show */
( ISPICKVAL(LeadSource, 'Trade Show/Event') && ISBLANK(TextField__c)
) ||
/* check TBP */
( ISPICKVAL(LeadSource, 'TBP Event') && ISBLANK(TextField__c)
) ||
/* Website has no other requirements */
/* check Other */
( ISPICKVAL(LeadSource, 'Other') && ISBLANK(TextField__c)
)

Note that I didn't have the name of your text field, so I used TextField__c; update for your field name.