Plates for 1d3 and 1ds3 are the same.
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I'd say that depends on which brand they are.
If using RRS (Really Right Stuff) plates, at least for the bottom plate, they are NOT the same going from the Mark I or Mark II to the Mark III.
When I switched to Mark III's I was disappointed that my plates didn't fit right, and after checking their site, found there were new plates for the III. I got those and they're SO nice! Perfectly machined, smooth comfortable edges, and perfect fit.