since we don't know the kids' height or torso length, this is what I would recommend for "typical" kids
-the radian is a good choice for the baby, it's slim, so good for 3 across, and will allow erf. get an angle adjustor to allow more room for the front seat riders
-for the older 2 I wouldn't get a radian. Most 8 year olds will have outgrown the harness by height, the highest slot is 17.5", and the 6 year old is probably close to outgrowing it too. Diono does allow you to go over the the top slot ff, but because the shell must come to the top of the ears or higher, most kids won't go much above the top slot.
-Most 8 year olds don't need a harness,at that age a harness isn't any safer than a properly fitting booster. I would suggest a recaro vivo, my son's fits very well in 3 across in my bff's santa fe (we had snugride 32, vivo, and safety 1st air protect booster). the graco turbobooster is narrow too, but I would say vivo is hands down winner for us.
-for the 6 year old, you may want to consider a booster too. Most 6 year olds are mature enough to sit properly in a booster. If your friend really wants to keep this child harnessed your best bet is a britax frontier, it has the tallest harness slots, and can make a good booster once the harness is outgrown. You guessing should be fine putting a radian, vivo, and frontier in a santa fe, especially if you do belt installs (allow you to put the seats closer to the doors). I will say, that it can be difficult buckling boosters in a 3 across, but eventually the older kids will outgrow a harness and have to use a booster, so it's doable.
