Watch out as tandems - with reversible seats, for the most parts - are the less spacious ones. Even with a reasonably tall seatback (like the Uppababy Vista's), the canopy is right over it, and the whole space is MUCH tighter. With a very tall 3yo, I personally would NOT go for a tandem, at least not with a conventional type of seat, whatever the backrest height - because you won't be able to fit him in. The tallest seatback I ever saw in a tandem was on the Quinny Hubb, to reply to your answer - I was able to fit a 3yo, even a 4.5yo in it, however the larger ones were a tad tight, and the steering got harder the heavier the child is (which is absolutely normal with tandems, and you might struggle with that with any tandem stroller you pick). If it had to be a tandem for you though, check the Baby Jogger City Select Lux - it doesn't have a tall seatback honestly, but it has the bench sear accessory, where there's no seat back. You can fit any child on the bench, and I know it's simple, but it will be, in the end, the only option that will last for your tall little one.
If I would be picking here though, I would go for a side-by-side for sure. Probably the Baby Jogger City Mini GT2 Doube because it has a large seat, yeah, and also a carrycot option for your little one. The push quality will be completely different, much better, and I get why you want a tandem that converts to a single, but honestly, the space and push are worth getting two strollers rather than one do-it-all (but not well enough).