1. How can we customize the tax fields? IN our country there are three parameters for the tax computation?1. How can we customize the tax fields? IN our country there are three parameters for the tax computation?

Thuis is not possible with projeqtor.
You may sum the taxes.

2. If set the tax structure for the customer,. will it appear in all the financial forms, reports such has quotation, billing, etc.

Each item has its tax value, but iyt is retreived, by default, from cusotmer value.

3. Once set the tax for a customer, can we modify the tax under project level without changing at the customer level?

Yes, you can change value on any item.