Law was cut on the lip by the blade but he managed to fight off Richardson. Cairns and three other girls ran out of the flat armed with wooden table legs and began battering Mather.
Eventually the pair managed to escape but not before they were slashed by Richardson.
Law was struck in the abdomen while Mather was hit on the left side of his chest and ear during the surprise attack.
Richardson and mother-of-one Cairns denied the assaults in November 2015 and went on trial at Hamilton Sheriff Court. A jury took less than an hour to convict them.
Giving evidence, Law said he had travelled to Hamilton for a party and had no idea he was going to be set upon.
He said: "I'd met Nicole before and we had spoken over Facebook to arrange this, we were both mailing back and forward.
"She sent the message first asking what I was up to and if I was getting on it.
"We were talking away and then I said I would come down with my pal and she said to come down.'
Law told the jury how Richardson had appeared waving a knife and demanding valuables.
He added: "This person then came flying out from behind the wall with a doctor's mask covering his face and he had a knife in his hand.
"He said 'give us your money, give us your drink, give us all your stuff' and was waving the knife in front of us and said I was getting wide on Facebook.
"I didn't know what he meant by that because I had no issue with him on Facebook but he went towards me and cut me on the face.
"I saw an opportunity and grabbed him by the shoulder and forced him back and was trying to get out.
"I turned round and saw three or four girls standing with wooden table legs and Connor was bent down trying to cover his head because they were hitting him with the legs.
"They had come down from the flat upstairs and I didn't know who they were."
Sheriff Douglas Brown deferred sentence for reports.
Cairns was granted bail but Richardson was remanded into custody.
Sheriff Brown told him: "Given your serious and significant record, you pose a substantial risk to the public so bail must be revoked."
Cairns and Richardson were convicted of attacking and robbing Law and Mather.