3 owners on a 5 year old car is not a lot. My 52 plate S had 2 owners and when that hits 5 years will be sold to someone who will be buying a 5 year old car thats had 3 owners. I todays age many folk change there car every year (thats what my cars first owner did as he upgraded to the
JCW), then theres large numbers that change their car when the 3 year warrantee is up (thats what happend to mine with the second owner) and when mine gets to the end of the TLC and I have some insurance I will be considering changing to a different car or model of MINI depending on finances and family circumstances (that will be about the 5 year mark). That easily makes 3 owners in 5 years. My car is pretty sound and engine wise seems to be a real peach.
On a 51 plate ONE or Cooper I suggest listening and feeling the clutch and gearbox as both were known to have issues on earlier cars. Leaking expansion tanks and trim rattles were also issue. Early cars also had something about pulling to the left while driving and under brakes, but most were fixed. Check that its gone to the dealers for all remedial work on the recalls issued on that age and model by BMW.
Get a decent test drive in it and have a good look round. Have a listen to the engine and check the basics like on any car (water i the oil, blowing exhausts colours of plugs etc. If its from BMW it will have a 1 year warrantee anyhow so minor stuff will be fixed without questiuon by a good BMW/MINI dealer.
R