MyDoom, Zotob, and Blaster are all examples of what type of malware?

Trojans masquerade as legitimate software to trick users into executing them; none of the three listed malware families used this delivery mechanism as their primary propagation method.

Kernel-mode rootkits operate at ring-0 to hide their presence on a compromised system - a persistence and stealth mechanism, not a propagation class.

All three are network worms - Blaster exploited MS03-026 (RPC DCOM), Zotob exploited MS05-039 (Plug and Play), and MyDoom propagated via email and peer-to-peer networks. Worms define themselves by autonomous self-propagation across systems without needing a host file.

Browser Helper Objects are DLLs loaded by Internet Explorer to extend browser functionality and are used by adware or spyware, not by self-replicating network malware.