On each Spark you can bridge the two interfaces but you cannot bridge those bridges at layer two and it sounds from your post that you are confusing layer two and three (ARPA).
So if you have three Sparks, you have three two port switches. Take three two port switches and connect them in a ring and you have a collision storm. You have to use STP or similar to sort that out.
The ring is now collapsed to A-B-C (with no A-C) and to get from A to C you have to go via B which halves thoughput for those two paths: A-C and C-A.
Another "option" is to define two VLANs and use MSTP and layer three and hope the protocol that shuffles data can route packets effectively. So you get two sets of paths at layer two: A1-B1-C1-A1 and A2-B2-C2-A2 and you cut the two graphs at different points, so you get two disjoint graphs, for example: A1-B1-C1 and B2-C2-A2. However they will still overlap and it all falls apart, somewhat.
You can only link two Sparks together without a switch to get the full possible throughput. That is why Nvidia only offer a two pack option. Three or more requires a switch to avoid a path being used twice. Ethernet is not Token Ring! Mind you Tring was 16Mb/s - luxury when ethernet was 10Mb/s 8)
I've also had this discussion with folk when it comes to "hyper converged" virty systems: So we have three nodes with two NICs each for Ceph and we cable them up in a ring and it goes weird ... . At least there I'm only generally specifying 10G.
Now, do you really need the full 200Mb/s for clustering SParks? No idea yet, I've only got one and I've only got 40Gb/s QSFP+ to play with on my switches.
However I'm seeing value in the one box already at the moment and if that scales then I'll be buying more Sparks and a really funky switch when I need three Sparks and perhaps another switch down the line with some really fancy VLT or whatever wankery is the stacking du jour thing!