Ok I'll play as I now have all 3 so can so some first hand comparisons. They are rather more different than I previously assumed and all 3 definitely have their own personality. I will ignore the Speemaster for the moment since that has been already discussed at length but the RM has been cruelly over looked to my mind. I had wrongly assumed the RM was just the SM design rehashed with a plain bezel, not so. It is noticeably thinner, the caseback has a very different profile and the crystals are totally different in curvature. The dial area may be similar or even identical but the RM design looks less heavy as the rehaut is shallower. I love the curve of the RM crystal, it is a real dead ringer for acrylic and gives no distortion or milky ring. The RM is definitely one that works better on the wrist than pictures would lead you to believe.
They do seem the same lug to lug and the overall width seems identical too, or at least within a fraction of a mm. The supposed 1mm difference may be partly down the the slight bezel overhang, in a similar way to how the FOIS and Trilogy Speedmaster are quoted as different sizes, but it is all in the bezel! The RM is noticeably thinner and snugger to the wrist at circa ~12.5mm vs the SM ~14mm. I didn't expect this and like it a lot. It feels like a military watch, like a slightly bigger WWW or Dynamic Gen 3 in many ways.
The bracelet which is the same on all 3 other than the end links feels least successful on the RM where it does show its size and heft. This is the one most likely to stay on the leather strap IMO. The bracelet works better on the SM as the overall watch thickness disguises the heft more. Below are some comparison pics that hopefully show these points. It was mentioned elsewhere that the RM dial may be browner than the SM or SP. I am not seeing that here but judge for yourself. The light here is very strong and will wash out the dials to grey a little, in normal use they look darker, black almost. The lume seems applied into shallow wells on both, not as obvious as on a sandwich dial but it is definitely not applied straight onto a flat dial as it sits flush or even a little below. Note the caseback profile on the RM is more like that on the Speedmaster, that is to say a double bevel as seen up until the Pro models. The SM design is more organic. The crowns look the same size but is screw down on the SM and not on the RM. The crown on the RM pulls out very easily so it is a good thing that there is water ingress protection on the stem regardless of setting or running status.
To coin a phrase, fail boat? My arse!