Can I add spacers to my cassette and crank to fit 3" tyres? Can I add spacers to my cassette and crank to fit 3" tyres? The chain rubs on the tyre.

I’m running a hope hub with SRAM 8 speed cassette and saint cranks. Can I just buy spacers to pull the cranks and cassette out so the chain is away from the tyre?
Nope.

For the cassette: the cassette mounts on the freehub body. The position of the freehub body on is fixed on the hub. Besides, there is no room for the cassette to move outwards as it's inside the frame.

For the cranks: depending in what width bottom bracket shell you have, perhaps there might be a solution using a crank with a longer axle and a wider bottom bracket but you really don't want to do that because you'll end up with a very bad chain line alignment.

I'm actually surprised you have not run into lack of tire space between the chainstays before hitting the chain.