I've had my retainer for 3 months (nightly). I forgot my retainer for labor day weekend, will my teeth change?

You may find that the retainer takes a little more effort to go in, as you may have had a slight shift but not adversely where wearing the retainer will not be possible.  I would to sweat it.  Good luck

Most likely it will not adversely effect your bite or cause shifting, depending on how well your bite comes together. After a few days, you might feel it is a bit tight when you put it back, but that feeling will go away fairly quickly.Dr. Kazemi

Although every person is different, the chance of teeth shifting over a 3 day period is pretty remote. You may find that the retainer goes in with more force when you are trying to put it back in but it only means that the teeth have shifted and the retainer will nudge them back into their position.