Tier 1 general extension

Folks,

I am going to apply my tier 1 general extension with 2 dependents already in UK (spouse, child) in this month by post.
1. Do I need to make 3 separate postal orders or just one with 2000GBP as a application fee
2. My son is 3 year old so Is that alright if I sign on behalf of him? and what title I should mention for him Mr or Master?
3. In tier 1 general main applicant foam, section -7, do I need to mention number of passport as 3 or 1 passport in all application foam (1 main applicant, 2 dependants)?

Any help on this would be greatly appreciated.

Thanks