Ryzen 2400: will the GPU part of the APU still be usable for compute if I have a discrete graphics card installed?

As of now, Ryzen APUs won't coexist with GTX1080Ti^[1], GTX1050^[2], GTX1050Ti^[3], RX580^[4][5] and Vega 56^[6].

Even if it is fixed with updates from AMD, Ryzen APU only has 8 PCIE3 lanes for dGPU^[7], eg. PCIE3 lanes are split x8/x0 instead of x8/x8 (see detail pane of spec). You'd need an adapter to fit extra dGPUs, as per your intent to run 3 dGPUs in 1 rig.