Bicarbonate of soda and baking soda are the same thing. Its called bicarb in the UK and baking soda in the US. Baking powder is a mixture of bicarb (alkali) and cream of tartar (acid) plus a filler to keep it dry. When it gets wet the acid makes the bicarb work faster and more completely. I don't think any of them will sweeten tomatoes. Use a good tomato fertiliser.