Two of the horses found shot. A reward is on offer for information leading to a conviction.
The three comprised two stallions and a pregnant mare.
A yearling foal was found with his mother, unhurt, but nuzzling the dead mare in an apparent bid to revive her.
Late last week, the sherrif's office announced that three more had been found.
The animals had been dead for some time.
The Central Oregon Wild Horse Coalition reported that they may have been shot at the same time as the three in mid-March.
It set up a reward fund following the first three shootings.
In addition, the Humane Society of the United States is offering up to $US2500 for information leading to the identification, arrest and conviction of those responsible.
"Shooting these three magnificent wild horses was an act of vicious animal cruelty, resulting in a young horse being left alone and motherless," said Scott Beckstead, the society's senior state director in Oregon.
"We share the public's outrage over these killings, and applaud the Crook County sheriff for aggressively investigating this case."