There are no BDSM elements. Zeke does get tied up with some neckties and it's clear this is a part of the MM and then MMF relationship, and it is consensual because of offstage negotiation between Zeke and Eric so yay, but I don't really consider that BDSM. I'm not sure it even qualifies as kink. To the extent there is kink in the book, it involves sexual activity in public or semi-public areas, and the hilarity is increased because they always seem to be observed (dad with poopy kid trying to get into the family bathroom, other people in the parking lot, other people in the drugstore, etc.). If there is an erotic theme beyond the triad, it is Getting Walked In On. If that is cringe-inducing, you probably should avoid this. If it makes you giggle, you'll find it entertaining.
Leaf makes a really solid effort to explore how a triad might develop, using the series-of-random encounters as a way to depict the logistical difficulties and practical implausibility of establishing a triad while making it not feel like "work" (which, hey, this is a novel, it's supposed to be fun, so okay). Zeke and Eric have a longstanding, close relationship as friends, business partners and roommates, but each has been concealing un-acted upon bisexuality from the other. The early encounters with Carrie give them a chance to grope and kiss each other and once they get home and have a chance to talk about their omg are you going to punch me fears, they start catching up for lost time. If you're sitting there thinking, that's not really plausible/they're really stupid, well, Leaf addresses this by having the characters specifically call themselves complete fucking morons.
Finally, Leaf deals with the what-will-family-and-friends-think issue directly: they're all freaked out, ask a lot of questions, have some amount of prejudice or, in the case of one character's largely absent father, resort to physical violence. The characters use the usual array of arguments for their arrangement: hey, people used to say this about other arrangements and it was wrong then, we're adults, we're not hurting anyone, just because it's hard doesn't mean it isn't worth doing and when-you-love-someone-you-have-to-take-a-c
All in all, about as perfect as a poly novel gets, from my perspective. There are always things to complain about (I'd start with the scene in the truck, viz. failure to discuss condoms/birth control), and obviously my preferences are far from universal, but I'll certainly give Leaf's other ebook-on-Amazon a try and keep an eye out for her other work in the future.