Ok, so the funny thing about the HF antenna is… it’s on the bottom PCB. Even though that middle PCB has a silkscreen design on it that indicates you put the cards there, for implants you want them up against the actual antenna trace, which is on the top side of the bottom PCB.
On my proxmark3 easy I’ve removed my middle PCB which is more or less a spacer to keep the LF and HF bits from interfering with each other. You can see the antenna trace clearly.
Basically, you’ll want to flip the proxmark3 over and put the bottom PCB on your hand instead. Also, be sure you take advantage of the delay command;