Probably not gonna get a consensus out of Blues fans here, and most of us are probably thinking differently than the front office, but IMO:

Sanford should've been gone last offseason when it looked like he was a good player - I'd take anything I can get for him at this point and I'd value the 3LW opening he'd create more than what he'll get on his next deal;

the price for Blais can't be that high based on his performances - maybe a 2nd/3rd rd pick or something like that - but I'd like to see him get more opportunity here;

Sundqvist isn't really tradeable during recovery from an ACL, but even if he wasn't injured I don't think he would get traded even with expansion looming. He's a key role player and they always seem to struggle when he's out of the lineup.

So would something like two 3rds and a prospect for Blais and Sanford work?

I'd more or less agree with all of above. I do think the organization and fans significantly overrate Sundqvist's importance to the team (people say we should protect him in the expansion draft, which I do not understand at all). But I really wish we would have sold high on Sanford last offseason. I do think even this year's Sanford would be pretty valuable to someone like the Canucks though, he struggles with finishing consistently which can make him frustrating to watch but he's replacement level- if not sound in his own right - defensively and he's a clear upgrade over everyone in the Canucks middle six other than (obviously) Horvat I would say.

The Blues almost certainly aren't trading Sunny.

Blais you could surely get for a 3rd, hell maybe even a 4th. I feel like the organization isn't seeing the progress they'd hoped for from him.

Sanford is a funny one. As much as some Blues fans hate on him, and as inconsistent as he is offensively, Berube loves him because he does a lot of the little things that help a team win. He's one of our most reliable zone exit guys, for example. The guy just needs to be more consistent, hit more, shoot more. I honestly don't think they'll trade him, but I see some AGMs here offering like a 2nd for him. That sounds crazy high to me, but if the Blues really get offered a 2nd for him (or say, someone like Jet Woo), he's gonzo.

Its probably a situation where you move one or the other. Already gonna lose a guy to Seattle and the only internal guy really pushing for a top9 F spot next year is Kostin. Not much cap space either, depending on how it shakes out.