generally updates replace the previous version ... so there is no need for 1 if you install 2 etc - however .net can be a bit odd ... some apps "know" what versions are available and follow the rules .... some will test for a particular version ... and if it's not there will refuse to work even though a later version is installed.
so the answer is .... possibly - it depends on the software you use
you need to suck it and see (what the hell does that actually mean??? is there a link with grannies and eggs???).
I have a couple of .net testers ... and they both report the version
S that are installed not the latest one.
and microsoft says ".NET Framework 3.5 builds incrementally" - which I read as needing the others
http://download.microsoft.com/download/9/a/e/9 ae0f6cc-7032-408e-9ca7-989f9e4af4ec/dotNetRead Me.htm