I don’t have an R3 but another Cervelo, the ancient SLC-SL. I don’t think their geometries have changed all that much since then. I’m 178 cm and my inseam is 83 cm if that is of any help.
I remember having the same issues as you regarding size, chosing between a 56 or 54. I finally decided on the 54 as it is the “pro”- way😜 seriously a to large frame is always to large and it is easy to just raise the saddle and fit a slightly longer stem on a small frame without it looking wierd.
I have long femurs so my saddle is far back. I used to have 120 mm stem but nowadays I use a 105 mm long. My handlebar is Richey SL ergo bend so it it low in the drops but not extreme. I’m 56 and not as flexible as I used to be.