To give a simple to understand, western comparison, Taobao is eBay, Tmall is Amazon and 1688 is Amazon Business.
That means Taobao is customer to customer (C2C), Tmall is business to customer (B2C) and 1688 is (B2B).
In reality, especially when you use a shopping agent, the differences are small. You will find many sellers on Taobao are well organized companies.
The only real difference this could make is that 1688 sometimes has minimum order quantities.
In terms of size, in our scene taobao is the clear number 1, 10x the size of number two, 1688 and then tmall not far behind.