Hi Maria,
That's correct, they take 3% of the total payment to you not including their service charge i.e Total payment x 3% = Payout to you (£100 x 3%=£97); they do round up fees to the nearest pound as well so sometime you might note it is slightly more that 3% i.e., £3.75 rounded to £4 or like in the case of my recent booking £5.04 rounded down to £5.
See this link - https://www.airbnb.co.uk/help/article/63/what-are-host-service-fees
Also, their new Terms and conditions was released today to existing Host who registered pre March 2016, I was on the bus so had free time to read through it before confirming/ agreeing to the terms and it confirms within this new terms that they round up fees.
Hope this is of some help.
Ama