Forum Moderators: skibum
One bad thing about Burst is they requisition a cetain percentage of your ads for their own personal use. I wouldn't mind but their personal use invloves slapping some really ugly banners on your site (for surveys or to advertise the network). For a media company, they sure went on the cheap to get those made!
So yes, you should definitely sign up with all three, then use them in what people call "a chain" -- i.e. set one of them as your primary, then a second one as your default, and the third as your second default. This way, you'll never get defaults, and every time one network runs out of banners to show on your site, it'll show your default ads instead. This way you'll always get paid for impressions, and won't be showing PSAs (public service annoucements that doesn't pay you anything).
If you have enough traffic, you should be able to get in with all three relatively simple. If you have A LOT of traffic, you might be able to get into Tribal Fusion. I'd recommend signing up for all four and see who takes you.