Yes

All service packs are cumulative

Pakled,

If you use LiveUpdate, you will get the SP's and hotfixes in order, though if you manually download them, you only need SP3 and then any hotfixes that are not included in that SP.  Be aware also that if you install SP's and hotfixes on your BE server you will also need to update any applicable agents you have running - each agent type is a bit different - Windows agents can be push-updated using the RAWS bulk update utility, but Linux-based and DLO agents need to be done manually on those machines.  There is not always a hotfix or SP necessarily needed for agents for every one patched to the BE server itself, but check the following list to be sure - Just put in 12.5 for your version and specify which agent you want.

It's fairly common for SP's / Hotfixes to break communication with agents, so make sure to figure out all the updates you will need on each machine beforehand if you have a critical backup setup and/or many machines to do.

http://www.symantec.com/business/support/downloads.jsp?pid=15047

You may also want to re-install the Symantec device drivers.  Go to Tools ---> Wizards --->  Device Configuration Wizard to do so.