For two curtains not three, I think metal is easier (three curtains tends to block more light in day time, and probably harder to avoid gaps at night). Bay windows vary a bit, and I've seen two systems that seem fairly flexible to different designs and sizes of window:
Metal track that you bend over your knee to match the angle on your window (you can find videos on youtube demonstrating this). These are quite rigid, smooth running and fairly quiet. Because the track is easy to bend by design though, they need quite a few supporting brackets, which some people might find ugly (especially the "double" ones designed for two sets of curtains running in parallel).
Metal pole and flexible joint systems. You can get "C rings" and brackets with C-shaped supports that allow the "rings" to pass over the brackets, so you can still use 2 curtains instead of 3. Unbranded parts based on 28mm diameter poles are readily available in the UK. For a 3-section bay, you'd get three metal poles, cut them to size with a hacksaw, and join them with the standard flexible joints designed for this purpose. Wherever the C-rings need to run past, use the C-shaped brackets. The C rings are usually metallic in appearance but may be made of plastic to reduce noise.
Depending on the window structure, you may find you have little room for enough overlap of curtain past the window edge to avoid a small gap at the sides. To get the curtain to stick to the wall to cover the gap, I used a ring magnet attached to the wall with a screw through the hole, one on each side (a Neodymium magnet 10 or 20 mm in diameter and a few mm thick has around the right force for this if the curtains are a mm or so thick). I sewed the steel "keepers" that came with the magnets into the curtains. You may also find you don't have room for 'finials' or certain end bracket designs.
Consider curtain length: if longer than window, will it drape oddly if the sill projects further than the pole/rail brackets? This might determine the choice of brackets, though it's probably easier just to pick short curtains that stop just above the sill.