Oh, I misunderstood.
You're right that there is only one tether anchor back there. It IS possible to safely do 3 across in the 3rd row of a Caravan with kids the age/size of your kids, but it means rear facing the 2 year olds and loading them through the hatch. Then in a year or so, the oldest would need to be in a booster, the twins (are they twins?) could both forward face, and the baby would have to rear face in the 3rd row, until the twins were booster ready.
Let us know if you're interested in that option. It would mean 3 new car seats now, plus a couple of boosters down the line, but all of the required car seats could be under $100 each, most well under.
If you're really interested in an Odyssey, though, I highly recommend it! I have a 2013 and "only" 3 kids. If you get a 2011 or later (they haven't changed the basic body design, including seats, since then), there is a tether anchor for every rear seating position, plus plenty of really usable lower anchors. And the 3rd row is way wider and less finicky than the Grand Caravan's.
For 3 forward facing harnessed seats across the 3rd row of a 2011+ Odyssey, one option that I'm certain would work is an Evenflo Sureride in the center (is that what you have right now?) and a pair of Graco Tranzitions/Wayz (same seat/different retailers) outboard. Or, I suppose 2 Surerides outboard with a Tranzitions/Wayz in the center would technically work, but it would be pretty claustrophobic in the center. The Sureride is fairly wide, but sits up on a narrow base, while the Tranzitions is much lower profile, so it fits under/behind the Sureride.