Question Traveling and 3 across

Hi!
I'll be traveling alone with my two little guys soon, and they were nice enough to approach their current seat limits simultaneously so I'll be inexperienced with their seats in lots of new places!
Oldest is almost 4 years, at 34 lbs and approximately 40 inches. I got him the Britax Frontier 85. Youngest is 7.5 months and will graduate into big brother's Britax Roundabout (and is approx. 28" and 21 lbs) - I'm experienced with this seat but RF was a while ago.
I figure these two will fit nicely in my car (2001 Jetta) and others, and I will get lots of practice quickly. My question is about when we drive with my sister (whom I will be flying to visit, and she will be flying back to visit me) and HER 7.5 month son and his new Roundabout 55. Will these three nest well in a back seat, obviously 2 Roundabouts in RF position, and I assume the FF Frontier in the middle? Would they work in my 2001 Jetta, or would it be better to borrow my husband's 2002 Chevrolet Trailblazer, or will we need to rent a car to transport my sister, me, and our three little ones? Similarly, would all three seats fit across in her Honda CRV (recent model - I think 2010)?
Thanks!!

If you'll be traveling often while needing 3 across, I'd recommend the ride safer travel vest for your oldest. Use it in the middle of the 2 rearfacing seats. With the rstv, 3 across is easy in most cars.

The RSTV is one good option; however, you wouldn't be able to use it for your oldest on the plane, and he likely would be more comfortable riding in his car seat on the plane. I'd suggest that you also consider the Evenflo Maestro, which is a forward-facing only seat that harnesses to 50 lbs and is quite a bit narrower than the Frontier 85, so would make it a little easier for you to fit your three across.

Using the seatbelt to install the two outboard seats will also give you more room for the center seat, because LATCH anchors tend to be a little closer to the center of the vehicle than the seatbelt anchors.

Since I neither own nor have access to any of those specific vehicles I'm not able to test fit them for you, but hopefully someone else with hands-on experience will be along soon.

Thanks! I had already purchased the frontier for the added harness height since my boys tend to be on the talker side, buy I'll keep the maestro in mind when we replace the seats in my husband's car. I love the vest idea too. I think we will end up traveling with both to maximize our options. I hate spending more, but it is cheaper than renting and driving a second car!
Thanks for your help.