Hi Martin,
the 3.5 arms are the same as Mi3. Mi2 arms fit, but don't have the swaybar mounting holes, these have to be drilled if you want to use Mi2 arms.
Mi3 CF arms are U3127 front, U3128 rear, these are the hardest ones you can get, I use them most of the time. Mi2 CF arms are a bit softer, some people like the U2765 Mi2 front CF arms on Mi3 (and drill the swaybar holes). If you need even more flex, there's U3249 Wishbones Front; Med Flex - Mi3 and U3250 Wishbones Rear; Med Flex - Mi3. On asphalt, I use the U3249 up front and original U3128 in the rear mainly, but on carpet I use the original U3127/U3128 most of the time.
The spool is the same, U3225 CNC Spool Assembly HD - Mi-3, but the things that wear & break are the spool outputs, U3141 HD Spool Output - Mi-3. Mi2 original spool will fit, but is physically weaker and also breaks the blades more easily. The U3225 spool fits Mi2 as well.
Most of the parts are actually the same between Mi3 and 3.5, they have just taken the most important hopup parts for the Mi3 and called it 3.5
Between Mi2 and 3.5, there's not much in common... body posts, rear belt and most of the bearings are probably the most important ones
Some parts will fit (like driveshafts and lower arms), but are a bit different.