Three aircraft below the transition altitude, A is 7000ft above 1013 pressure level, B is 7000ft above 1000 pressure level - the current QNH.
A's transponder will be sending 7000 to radar, B's will be sending 7390. These figures are then corrected on ATC display to show A as being at 6600 (his actual altitude above the QNH pressure level) and B at 7000.
Aircraft C is opposite direction at 6000 on QNH. Good chance for breakdown of separation with A, as A has the incorrect pressure set on altimeter.