preparing return where property sold was owner occupied rental. is tax payer allowed sale of personal residence? property a triplex do I use only 2/3 of Selling price on form 4797. First time had a owner occupied rental sold. Would appreciate some di

It sounds like it is NOT an owner occupied rental.  It sounds like the owner lived in one "dwelling unit" and had two other "dwelling units" for rent.

If all units were equal size and value, then the owner's portion is 1/3 and the rentals are 2/3.  But if they are not equal size or value, then you may need to adjust the percentage allocated to each.

Does that help at all?  If not, please post more details about exactly what you are asking about.

And why did he not receive a 1099-S?  Did he lie at closing and sign a form saying it was all his primary residence?  (Sometimes people sign these without reading them.) Or maybe they sent one but they didn't have his new address and the forwarding order expired.  If there ever was one. 

Did he live there 2 out of the last 5 years?  An interesting question would be if it made a difference if he moved from one unit to another.  Glad you didn't ask that.  

The rental portion goes on the 4797, and the personal portion goes on Schedule D, perhaps claiming the Section 121 exclusion.  As @TaxGuyBill points out, the allocation between rental and personal depends on facts and circumstances.