A triad is all three people involved romantically with each other. They may like YOU. They may not be interested in EACH OTHER.
I would say the best approach is to state what you want to person A and person B in person or by email separately and offer to have a meeting in trio later if they are both up for it to clarify expectations of each other/their roles in the possible polyship.
Finding out if they are even willing to just DISCUSS. Could be they are willing but each envisions a different model. So... not a runner. But you come to that square 2 only after passing square 1. So deal with square 1 first.
"Listen -- I really like you. I want to date you and (other person) at the same time. Is that something that could interest you and you could be ok with? And if so, what kind of open model relationship would interest you the most? Would you be willing to get together in trio to talk about details like that and what it could be like if we all decided together that we want to go there?"
They are either up for it or not.
The only way to KNOW where they stand with their willingness is to actually ask them. You can't be a mind reader. They could say yes, willing. They could say no, not willing.
Either way, you get to get to KNOW. Then you are free to choose your next behavior from a place of full information. You could choose to date both, to date one of them only, or date neither of them and seek a triad elsewhere.