ABE Controller experienced a loss of separation event when corrective action involving two aircraft was too late to secure needed distance; the reporter noting the event occurred shortly after assuming the position.

Narrative

Air Carrier Z was a factor in this scenario; but I don't believe that separation was lost between that aircraft and either Aircraft X or Air Carrier Y. Air Carrier Z was approaching ABE from the west to land at ABE. Air Carrier Z was descending to 6;000 feet east bound while Aircraft X was northeast bound climbing to 5;000 feet. The speed of these two aircraft led me to believe that Air Carrier Z would overtake Aircraft X and then be able to continue descending once Aircraft X was overtaken. As Aircraft X leveled off and the airspeed increased; I also observed that Air Carrier Z was slowing down. I called traffic to these aircraft; and upon observing the traffic Air Carrier Z was instructed to maintain visual separation with Aircraft X and descend to 3;000 feet. Air Carrier Z asked about the heading of Aircraft X; and upon being told that he would continue northeast bound; said something about passing him to the south. While this was occurring Air Carrier Y was south of them north bound at 4;000 feet. This aircraft was going around some weather that I could not observe; and would continue north bound to the LVZ VORTAC when able. I became concerned about Air Carrier Z descending to 3;000 feet in time to remain separated from Air Carrier Y. I climbed Air Carrier Y to 5;000 feet to help insure that these two aircraft remained separated; and this worked as planned. The problem was that Aircraft X still remained north of Air Carrier Y at 5;000 feet; and now Air Carrier Y was also at 5;000 feet. I was not yet aware that these two would conflict; and I proceeded to point out the airport to the landing Air Carrier Z. Once Air Carrier Z was cleared for a visual approach and switched to the Tower I went back to the other two aircraft. When I observed the approaching conflict between Aircraft X and Air Carrier Y they were 3-4 miles apart and converging. I turned Air Carrier Y and called the traffic. I turned Aircraft X also and called the traffic. Air Carrier Y reported the traffic in sight and I instructed him to maintain visual separation. I told Aircraft X about this instruction to Air Carrier Y. Unfortunately; I believe that these two aircraft were approximately two miles apart by the time that this was accomplished. Aircraft X was returned on course once the conflict was averted and neither pilot expressed any other concerns.My entire career I have heard that errors are very common at the beginning or at the end of a shift. This event occurred eleven minutes after the start of my shift. My first mistake was not vectoring Air Carrier Z away from Aircraft X for his descent and the original separation. I knew that the overtake would need watching; but I chose to let the aircraft continue straight towards the airport. My second mistake was when I climbed Air Carrier Y to 5;000 feet. I was focused on him and Air Carrier Z at that point; and somehow just wasn't thinking about Aircraft X at 5;000 feet. I did need Air Carrier Y at 5;000 or 7;000 feet prior to him leaving my airspace and I was probably thinking about this when I climbed the aircraft to insure separation with Air Carrier Z. The need to clear Air Carrier Z for an approach contributed to my inattention to the problem about to occur with the other two aircraft.