It will neither positively nor negatively effect IQ.
IQ doesn't work that way.
From what I remember reading from a textbook, growing up in a multilingual environment temporarily slows a child down a bit in the first couple of years in school however the child does catch up fairly quickly before the end of primary school.
This is nothing to worry about and will actually benefit the child later in life.
From conversations with friends who have raised children in bilingual environments there may be moments of confusion but it eventually works itself out. I've a friend whose daughter was raised speaking both Greek and English. A teacher asked her a question in English in the fifth grade and she answered in Greek because she was unaware that the two languages that mom and dad spoke were different. This same girl (now much older) just finished a Ph.D.
Expect some confusion but be patient and don't over react. It'll work itself out and the child will grow into an adult who will be fluent in three languages.