When the first guilty verdict was read, Watson turned to look at his attorney and looked shocked. Then he faced forward and lowered his head a little.
There were quiet sobs in the courtroom when the verdicts were read. Some family and friends of all involved were crying.
Watson, who pleaded not guilty last year, could face life in prison on the first-degree murder convictions and 10-25 years on the second-degree murder conviction.
First-degree murder is punishable by death or life imprisonment while a second-degree murder conviction can result in a prison term of 10 to 25 years.
Watson's first murder trial ended with a hung jury late last year.
The second trial lasted eight weeks.
Watson was convicted of killing his ex-wife Linda Watson in August 2000 and his mother-in-law Marilyn Cox and her friend Renee Farnsworth in May 2003.
"We're not vindictive people," said Pat Hinkle, Cox's sister. "We just want justice for our family and I take no great pleasure in seeing anybody have to spend the rest of their life in prison, away from their family, but these are the choices he made, the bad choices he made."
The prosecution had argued Watson had the motive and opportunity to kill all three women.
David and Linda Watson were in a custody battle over their daughter Jordynn when Linda disappeared. Marilyn Cox was in a dispute with him over visitation rights when she was gunned down.
The defense had said the prosecution's key witness, Watson's second wife Rosemary, lied and there are more likely suspects than Watson.
The defense also claimed the prosecution's case is weak and the evidence is all circumstantial.
"We're shocked that you could take the same evidence that came back with a 10-2 vote and go 12-zero the other way without presenting anything new and follow the law," said Richard Johnson, a long-time friend of David Watson.