So I was wondering if it was possible to re-space a nexus inter3 hub to 135mm to fit my mountain bike? Or can the Sturmy Archer S-RF3 be re-spaced to that size? Or would I just be better off getting the nexus SG-3D55 3 speed disk hub? I ask this as I dont know the durability of the new SA hubs. Thanks.

You need to make sure the axle length is sufficient for respacing. If it is, you can add spacers to it.

If the frame is steel, narrowing to fit the hub is also an option. Steel can be bent slightly inwards or outwards without weakening the structure.

Like relyt said, if your Shimano Nexus three-speed is the model with the 189.4mm axle, you should be good to go. If not, well, Shimano doesn't make individual parts available for their IGHs, so probably not.

Yes, plus it's available in long axle length models and the S-RK3 disc brake model is available as well. I'd recommend Sturmey's RS-RF3 hub with its inboard-the-dropouts rotary gear select and solid axle (the only three-speed with a solid axle) OR the SRAM iMotion3 Disc hub, also with inboard gear select.

If you have some hard information that the SG-3D55 is more robust than the S-A hubs, please share!

Best,
tcs