I have a 2004 7 pass. Sienna, and I am not certain how the second row of your van compares with the third row of mine in overall length. I have had 3 seats in the back row, but they were all combo booster seats that tend to be a bid narrower than convertibles. Infant seats tend to be narrower than convertibles. How much "room" do you currently have betwwen your evenflo seats?
In terms of safety all seats with a 5 point harness are going to be extremely safe assuming you install and use them correctly. You may have to go to a store like Babies R Us and just try the seats in the van to see if they fit well together. You may find that two children get one seat and the third gets a different one-it is a puzzle sometimes! While the rear-facing tethers are nice, they aren't really "needed" since rear-facing seats are inherently safe. The rear-facing tether is helpful in rollovers and side impact crashes, but side impacts are not particularly common (the new Sienna gets great side impcact crash scores) and roll overs are not too common in a minivan. If I was driving a high profile SUV with a sign. roll over risk I would want a RF tether though.
The Britax Wizard and Marathon have a 49 inch, 65 lb forward facing limit so even though expensive, should last 5-6 years. While the seats are very tall, and somewhat wider in the seat area than a traditional convertible with a 40 lb/40 in limit, the base is pretty small, and seems to fit well in many places. For example, I have a Wizard in each of our Captains seats and the base is much smaller than the seat itself, so it may be possible to fit three across. Another advantage to the MArathon/Wizard is they are very easy to tighten the harness when rear-facing, and that may be a consideration, especially for the seat in the middle of the second row which may be hardest to access.
