Canon 1Ds MIII has a 21.0MP Full frame (36 x 24 mm ) sized CMOS sensor . On the other hand, Canon 1D X has a 18.0MP Full frame (36 x 24 mm ) sized CMOS sensor and features Dual Digic 5+ processor.
Below you can see the 1Ds MIII and 1D X sensor size comparison.
Canon 1Ds MIII and Canon 1D X have sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture. On the other hand, since Canon 1D X has 17% larger pixel area (48.21µm2 vs 41.08µm2) compared to Canon 1Ds MIII, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.