Ans to Q no.1. Dear Saifi, You need to pay GST under RCM for GTA Service availed.
Ans to Q no.2. If your supplies falls under more than 5% tax bracket it is advisable, ask your buyer to arrange the transporter for transportation of goods, otherwise the freight also will be the part of your taxable supply.
Ans to Q no.3. It is composite supply and supply of goods is a principal supply, therefore the rate of tax on freight will be the rate of tax of your principal supply.