I get to be the first to welcome you aboard.
Just from what I've read so far, it seems like you've been admirably careful and conscientious about things, and Steve has been consistently encouraging toward you. So, I would suggest (and I know I'm just an internet stranger but) that you treat yourself to some romantic alone time with Tim.
As for whether you're poly or just free-spirited, maybe you're both.
Don't worry too much about fitting the right label, just enjoy the good things you have and don't be afraid to pursue happiness.
Has Steve ever wanted to be with another woman? Just curious.
Regardless, I'm glad to have you with us on Polyamory.com.
Many well wishes,