Question 3 car seats in Buick Enclave

Hello,
We are seriously considering purchasing a used 2009 or 2010 Enclave to accommodate our growing family (7 seater...so 2 captains chairs in the 2nd row). We have a 2 year old and twins on the way. Does anyone have experience with 2 infant carriers and 1 forward facing convertible car seat? Specifically we have a Britax Marathon and 2 Graco Snugride infant carriers (still new in box so can be returned). Here are my specific questions:

1 - where/ how should I place them in the car (Gracos in each of the captains chairs (LATCH) and Marathon in the center of the 3rd row (anchor and seat belt installation))? Will this combination of seats fit?
2 - has anyone put 2 convertible seats in the 3rd row? If yes, are they on the sides and only secured by the seat belts or do you anchor them to the center anchor? Sometimes I take a friend and her 2 year old daughter along with us.
3 - I'm not opposed to returning the Gracos and buying something that is safe but a better fit. I'd also purchase a new convertible seat if that is the safest option...safety come first!! What seats would some of you recommend for the Enclave?

Many thanks! Getting a car has never been so stressful!

From looking through an older post regarding the enclave, it appears that the Enclave only has a limited number of tether anchors on that third row (only in the center) and no latch anchors in that row. If you are wanting to install a forward facing seat not in the center position, it would have to be untethered which really isn't recommended.

For seating arrangements, I would probably do do the two infant seats in the back and the marathon in the front utilizing the second row tether. This will leave the other 2nd row seat free for your friend's child.

How much does your 2 year old weigh? If the child is still under the rearfacing weight limit on the seat, moving that seat rearfacing might give a few more seating options.