I would return the 3in1 seat as well. It will not be the last seat you ever purchase AND with it harnessing to only 40lbs, you can find a seat for the same price that will harness longer.
I have an 03 Tahoe and when installing a seat
RFing you will have to use a pool noodle. I have tried SEVERAL seats in my Tahoe and haven't found one that wasn't compatible.