Yikes, it could be difficult.  Boosters can actually take up more space than harnessed seats, since you need enough space to easily access the buckles.  I see, though, that your middle child is only 3 or 4.  That is definitely too young for boostering, and a harnessed seat is a must.  You might have luck with a narrow booster for your oldest (like a Turbobooster or Parkway), a Radian for the middle child (it is the narrowest harnessed seat you can buy), and a narrow seat for your newborn.  If I remember correctly, the Focus wagon does not have headrests in the back seat.  That means you MUST keep your oldest in a high back booster. Â
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The narrowest infant seats are the Chicco Key Fit and the Baby Trend Flex Loc. Â The Flex Loc handle must be down in the car, though, which might make it feel like a tight front-to-back fit in a small car, if you're constantly needing room to move it up and down. Â An alternative to an infant seat would be to get a Coccoro for the newbie. Â It is a small, narrow convertible that fits newborns well and often takes up less space than an infant seat. Â
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Search this site http://www.car-seat.org/showthread.php?t=33226 for others that may have had luck getting 3 across in a Focus.