I've fit my three in the back seat of our Passat. Well, sort of. We have 2 booster aged kids, and one infant carrier aged. Our usual set up in the back is one booster and an infant base, and a second booster carried in the trunk. For comfort, we can take up to two adults in the front, plus any combination of 2 in the back.
But three in the back can be done. The carrier must be installed without the base in an outboard location, and the two backless boosters can be squished close together. Seatbelt access will be tight.
My oldest son is on the cusp of going booster-less. We have come home, well to their bus stop a couple of blocks away, with our youngest in his carrier with the base, to meet them coming off the bus. All three fit no problem, with one booster, one carrier with base, and one without a booster.
Normally, if the 5 of us go somewhere, we use our van instead, but the Passat if it's a smaller load of passengers.