If you want to query the name of the last user who logged on:
Computer.Login Name OR Computer.Full Name
If you want to query the last time a user logged in:
Computer.OS.Last Start Up Time
Keep in mind that these two attributes will only give you the information on the very last logon. If you want to keep track of the last 3 logons as you mentioned, you will need to enable the Inventory History feature on those two values (Configure > Inventory History)
We store this in the Registry on the client under:
However we only send to the Core Server the last logged in user as Computer.Login Name.
We also send up Computer.Primary Owner which is whoever has logged in the most in the configured amount of times. This is Configured in the Configure | Services | Inventory | Primary Owner Logins. Read the help file on this screen in the Console, it is extremely explanatory.