Python rounding final answer from a formula

Currently have a formula that outputs a number, with the round() function rounding the answer to a whole number. How can I get 3 decimal places in my answers.

Odd request, but is there a way for it to round down always, even if 5 should round it up?

orderSize = round((availableBalance * 15 * 1 * (1-(0.00075 * 2)))/lastPrice)

Use it like this orderSize = round(((availableBalance * 15 * 1 * (1-(0.00075 * 2)))/lastPrice),3)