Re: 3 accross
Are you buying all brand new car seats or what seats do you have already to work with?
What's your budget?
What's the shirt size for each kiddo?
The radian is a good narrow seat. But I wouldn't recommend one for your 5yo because the booster is outgrown by height at the same time as the harness and has a 50lb minimum for booster use. Since you're not going to RF the 5-year old, you'd be paying a ton of money for 1/3 of a seat.
The radian is a good narrow seat though that extended RF and harnesses. And it has a very low profile so it "puzzles" well with higher car seats.
If you are buying all new car seats, I'd recommend looking at:
Graco Size4Me (tallest RF shell on the market and 40lb RF limit) in the center for the baby.
Radian R100 RF for the 3yo on the driver side (cup-holder side of S4M). You can get an angle adjuster for $10 separate that will install the Radian more upright for a better fit behind the driver. What year Outlander? Is there a light indicator in the front passenger seat that turns on the airbag depending on the weight in the seat? If not, you can brace the Radian against the seat as hard as you need to. If there is an advanced airbag, you should be able to install the Radian lightly touching the front driver seat.
Evenflo Maestro for the 5yo. It will harness to a shoulder height of 18" or weight of 50lbs. The Maestro does become a high-back booster as well, but the booster is outgrown at the same time as the harness. So I'd harness as long as the Maestro fits (bonus for being able to test out maturity in booster mode before it's outgrown) and then get a Graco high back turbo-booster in a few years.
The S4M and Radian should last your younger kiddos until booster age/maturity.
Radian and Size4Me fit well side by side with both RF, but not so well with the S4M FFing; fairly well with the Radian FF and the S4M RF; and well with them both forward. So you may want to swap seats when you're ready for your middle to FF.
If you can't afford a Radian for the baby, a Cosco Scenera (RF for baby) puzzles well on the cupholder side of a FF Size4Me for the 3yo. [I hate to recommend a car seat that Rf and FF to be used only FF, but it should last your middle child to booster age.] The Maestro for 5yo then puzzles okay on the non-cupholder side of the FF Size4Me if you put some pressure on it for a tight install.
Maestro puzzles a little with the Scenera as well if you wanted to put the baby in the middle.