Even I didn't know about it a few weeks before the release!
Check out the trailer...
The plot is basically that the chipmunks feel scared that a wedding is on the way when Dave Seville's (played by Jason Lee) relationship with Samantha (played by Kimberly Williams-Paisley) is in full swing. So when Dave goes to Miami to propose his record to artist Ashley Grey (played by Bella Thorne) with Samantha, the chipmunks get onto a plane to follow him and sabotage his apparent 'proposal'. On the way there, they are chased by Air Marshall James Suggs (played by Tony Hale). Of course as you expect, Dave never intended to propose so the chipmunk caused so much trouble over nothing... AGAIN!
In my opinion, despite such negative ratings the film itself is great fun to watch. It is a perfect family comedy film which is agreeable to all ages of people. Definitely something I would recommend based on its predecessors!