if you already have SQL2008SP2 with some Cumulative Update Pack (CUP) you should not apply SP3

but now that MS have released the first CUP to SP3, you are good to go !

http://sqlblog.com/blogs/aaron_bertrand/archive/2011/10/18/cumulative-update-1-for-sql-server-2008-

sp3-is-available.aspx

so the list needs to add 10.00.5766 entry