I am 3 and a half weeks post transconjunctival lower blepharoplasty. How do I decrease puffiness at this stage?

Than you for your question. You are in the early stages of the recovery process where swelling and bruising are common after surgery, which cause the puffiness. I suggest that you remain patient and simply follow the post op care instructions provided by your board certified facial plastic surgeon.
Best wishes,

At this point of your healing some swelling is quite normal.  See your surgeon for evaluation, as some alterations in your sleeping position, warm/cold compresses or alteration in diet might be helpful.

A photograph would be invaluable to help us answer your question.  Please consider reposting with a photograph so that we can be more specific.