For starters, you asked for what is safest, and you will get this a lot here, b/c it IS what is safest--please, please, please turn those kiddos back around to rearfacing and keep them that way until they outgrow the 3 in 1s rearfacing (which isn't until 35 lbs and around 34-36" tall). The 3 in 1's have a rule that kids must be 34" to forward face (it's in the manual and on the side of the seat). The 3 in 1 seats make great rearfacing seats b/c they are fairly tall for 40 lbs convertible carseats and they have a high rearfacing weight limit. You can look up why rearfacing is safer all over this site--there's loads of information. Rearfacing them matters a million times more than where their seats are in the car.
When baby comes, it may be most convenient if you have a minivan, to have one twin in the third row either in the center or outboard, one twin in the second row behind the driver, and baby in the middle of the second row. Having the two second row seats pushed together gives you more access to the third row. Once the twins are forward facing again, you may want to put one twin in the center of the third row and one in the center of the second row, and then put the rearfacing baby outboard in the second row. You want the least protected child in the most protected position (least protected is the one who is forward facing or if you have a kid in a booster, the one who is in a booster, b/c rearfacing is safest). The most protected position is in the center of the vehicle. Once kiddos are forward facing, it will also depend on where the top tether anchors are, b/c you definitely want the seats tethered. Some vans only have ta's in the outboard positions or other weird set ups.
Congrats, congrats on the new baby--that's fabulous news!!! (Oh, and as I'm sure someone will chime in--we can't believe you are even thinking about going out with them all at once--you are a brave woman!)
henrietta