FWIW my worst and completely impossible carseat experience was in a Subaru Forrester. The seat is incredibly narrow, very contoured, and just a royal PITA. I don't know the similarities between an Outback and a Forrester... I think I have heard that the Outback can be easier but I can't remember where I read that. Maybe check the "successful 3-accross" thread and search for outback in it.
Honestly, If I owned a Subaru and was in your position, I would start looking into selling it and putting the oney toward a different vehicle, either a minivan or a larger, more car seat-friendly station wagon. For example. I was able to fit 3-across (not easily, but it was possible) in a 1990 Honda Civic wagon, but it's really a lot harder to fit 3-across in my 98 Saturn station wagon, and my sitter's '00 Subaru Forrester was the hardest car I've ever tried to put seats in. Check the 3-across thread for ideas about cars that are more 3-across friendly. I'm tempted by the Toyota Camry wagon and the Honda Accord wagon, but I have not owned or tested either of these cars so I can't vouch for car-seat compatibility, I just suspect it would work since they are larger/wider than the civic wagon and I was able to fit 3 in the civic wagon.
You might do fine with a FFing Radian in the center position and very narrow bucket seats on the outsides, but you would likely want to look into a different vehicle by the time your twins are big enough to switch to RFing convertible seats. You could potentially get two RFing Radians, but RNs are so tall that they can sometimes not fit properly and force the driver's and passenger's seats way forward leaving not enough space for the people in the front seat, so a RFing Radian doesn't always work. Other convertible seats fit better front-to-back and can be installed a bit more upright for older kids (I have the Marathon for this purpose) but seats other than the Radian are wider and I wouldn't be all that confident that you could fit two of them AND another seat for your DD in an Outback.
Because these just came out and go to 35lbs now, here is a plug for the Safety 1st OnBoard infant seat. THey have a narrow base, and since they go up to a higher size and weight, you would not have to deal with larger convertible seats until at least one year of age. There is also the Chicco Keyfit 30 that goes up to a higher size/weight than the average infant seat, but I am not sure of the width of the seat although it appeared to be narrow when I saw one in ToysRUs, I didn't get a close look.
However, many things are possible. If you get a workable 3-across configuration in your Outback, please post about it!
Good luck!