could you please tell me which of these is better?
1) apply to sb. for help - but you'd say this if you were applying for a top-up to your research grant or something like that.
2) address to sb. for help
3) refer to sb. for help (You can 'refer to sb for a second opinion', but that construction sounds a bit funny with 'help).
E.g. I address to you with a request for help.
Thank you very much.
I'd just say 'ask sb to/for help' or just 'ask for help'. Or if you want to specify who the sb is 'ask [for] Dick's help'/'ask Dick to help'.