Spot metering with one or mutiple flashes
The problem is that you want to blow out the background, i.e. overexpose. So if you're using TTL you've got to arrange the powers plus the overexposure amount so that you get good exposure where you want it, on the figure. Plenty of confusing interactions there!
It might well be easier to use the flashes completely in manual mode. Set up the portrait flashes by themselves to give good exposure, then set up the background flash by itself with enough overexposure. Then combine them and shoot. I certainly find that much easier. So it seems do lots of people working in studios with multiple flashes.