The grandfather-of-one has not been seen since June 23 in Hendon.
This morning, a 55-year-old man was arrested on suspicion of murder and two men, aged 59 and 39, were arrested on suspicion of assisting an offender.
Enquiries have been carried out to trace him, including viewing CCTV, carrying out house to house enquiries and speaking to people who knew him.
Chief Inspector Jerry Pearson, of Sunderland Area Command, said: "This is a fast moving investigation that has recently seen a number of significant developments that have led to three men being arrested this morning.
"Robert's family have been updated on this development and are understandably incredibly distraught by this. It is crucial we now find out the full circumstances of what has happened and we are continuing to ask for witnesses to come forward.
"If anyone has any information they think may help, no matter how small it may be, we want them to contact us. We will treat any information in the strictest of confidence and respect the anonymity of callers if this is a concern."
Anyone with information should contact Northumbria Police on 101 ext 69191 quoting reference 410 260614 or Crimestoppers anonymously on 0800 555 111.
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