I am curious why you think it must be a triad? Is it the husband who wants a triad mainly? Are you sure you want a triad? Could you want a V as redpepper suggests, where you date him but you're also FWB with the wife?
If so, you might want to think about things like lets say they DO find somebody in a year or two they both love and want to be in a triad with. Will you be kicked aside? Want to be in a relationship with them even if somebody else is involved? Do you want to date other people at all? Will they? From what people write about triads, they give a sense of security, but plenty of them end up wanting to change the dynamic at some point, so just make sure you think about what you want, and talk to them about their desires a LOT before rushing yourself into anything serious! If you can just enjoy the interactions you are having and take a lot of deep breaths, there isn't a need to do much more than communicate and enjoy right how probably.
Happiness will never come to those who fail to appreciate what they already have.