It happened on Thursday, September 20 at 7.25pm in an underpass off Harport Road between Lodge Park and Greenlands.
The victim, a man in his 20s, was approached by three Asian men, two coming from behind him and one in front who pulled out a Stanley knife and ordered him to hand over his valuables.
He tried to fight them off and stopped them from stealing his mobile phone but fell to the floor and suffered a cut hand.
The victim managed to run from the scene after the robbers had grabbed a small black bag from him which had long string handles. It contained his lunchbox.
The robbers were all thought to be aged between 20 and 24, around 5ft 6ins, of medium build and wearing hooded tops.
One of them was wearing a top with an urban camouflage design and one of the others was wearing beige trousers.
Detective Constable Dave Jennings of Redditch CID said: "Fortunately the victim's injuries did not require medical treatment but obviously he was pretty shaken up by this.
“It was a senseless robbery in that all they managed to steal was an empty lunchbox.
“However the use of a knife is very worrying and it is essential we catch these men.”
Anyone with information can call 101, quoting incident reference 562-S-200912, or Crimestoppers on 0800 555 111.