Here is a link to the sample I-129F form for K3:
http://www.visajourney.com/examples/INS-Form-I-129F-K3.pdfProof of bonafide marriage includes:
Note: Evidence of a Bonifide Marriage
The USCIS now requires that when filing an I-130 for a spouse that you include evidence of a bonifide marriage. They list examples of acceptable evidence as:
1. Documentation showing joint ownership or property; or
2. A lease showing joint tenancy of a common residence; or
3. Documentation showing co-mingling of financialresources; or
4. Birth certificate(s) of child(ren) born to you, thepetitioner, and your spouse together; or
5. Affidavits sworn to or affirmed by third parties havingpersonal knowledge of the bona fides of the maritalrelationship (Each affidavit must contain the full nameand address, date and place of birth of the person makingthe affidavit, his or her relationship to the petitioner ofbeneficiary, if any, and complete information and detailsexplaining how the person acquired his or herknowledge of your marriage); or
6. Any other relevant documentation to establish that thereis an ongoing marital union.