Radio installation

I ran into a problem installing my new car radio. First of all, I took the factory radio out, then got a new radio, adapter, and connector. I consulted the little book at the store to find out which adapter and connector I needed to make sure they fit, and they both said 1990 Buick LeSabre right on the box. So, I go home, try to install the dang thing, and the connector doesn't seem to fit the wire plugs in my vehicle. There are three different plastic plugs that house the radio wires that are blue, white and black. I tried to plug them into the big black plug of the connector, and they don't fit. Any ideas why?

" I consulted the little book at the store to find out which adapter and connector I needed to make sure they fit, and they both said 1990 Buick LeSabre right on the box. "
So was there two at the store? Do you have a premium sound system maybe with a factory AMP? I would suggest going to another store like best buy or something and looking for one.