For a lot of midsize cars, one typical 3-across arrangement is to have a narrow rear-facing carseat (like a Diono Radian, Clek Foonf or Combi Coccoro) in the middle seat. The Radian and Coccoro should even be suitable to use with a full-term newborn, though having them in the middle obviously isn't as easy for you to load baby into the seat! Then, you might even be able to get your current seats outboard next to the narrow one in the middle.
I assume your DD is still rear-facing? What about your DS?
Depending on the answer, another possible setup is to use try a Radian in the middle for one of your older kids, either RF or FF (for your 3 year old). You may need the Diono angle adjuster for rear-facing. Then, try a narrower infant carseat for the new baby on whichever side is easiest for you. A couple models known to be narrower than average:
Amazon.com : Chicco Keyfit 30 Infant Car Seat and Base, Sedona : Rear Facing Child Safety Car Seats : Baby
Amazon.com : Cybex Aton 2 Infant Car Seat - Charcoal : Baby
Then on the other side, you would use one of your Evenflo Triumphs, either FF or RF depending on who is in the middle and what puzzles the best.
I wish I could say if you could puzzle your two Triumphs with a new infant seat. It might be possible, but I'd need to see it in person. Perhaps someone else who is familiar with the Malibu can chime in on that!