H1B transfer, How to fill the employment date?

I work for company A for one and half years with H1B visa, now I get an offer from company B. So I need transfer the H1B. Anyone know how to fill the I-129 form about the employment date? I can start in July, but do I need to fill the date that ends my first three years of the H1B, which means I can work for company B for the rest one and half years, then later time do the extension, or I can ask for a three years during this application?