Do i need to file LCA again for H1B extension?

Your employer will know what to do. They should. Suggest they engage an immigration attorney. LCA will still be needed, for example.

None of that should be your concern or responsibility, and it's entirely your employer's. Anytime your current (or any other) employer seeks to petition for the extension of your current H-1B status, just like before, a new LCA needs to be filed by the employer and be certified by DOL. Only then will employer be able to proceed and file the H-1B extension petition with CIS, which in this instance, since you have an already approved I-140 will for 3 additional years.

You, the employee, can't file an LCA ... that is the property and responsibility of the employer's immigration lawyer. Keep in mind that the I-140 is also the company's property. What did the company's immigration lawyer say when you asked him/her these questions?

This should be handled by your employer and their attorney. You should not be dealing with the LCA process yourself.

If I-140 is approved, you will be eligible for extension beyond the sixth year and your employer will file an LCA to support their petition.