3% ROI isn't great from a cash perspective, but there's other benefits you're getting - those 100,000 impressions are potentially worth something for people recognizing your product later, then there's getting more reviews, potential word of mouth, etc. Also, someone that checked it out first from the ad might buy later, especially if they get repeated exposure.
The danger with such a low return is that if you're not watching carefully, you could run in the red pretty quickly and lose money. But as long as you're above break even and paying attention, your real ROI is probably a bit higher than that 3% considering the other positive effects.
Edit: Wait a minute, you got 4 sales... do you mean you spent $100 and got $3 in return? I was thinking you got $103 in return... if you're in the hole $97, that's utterly abysmal and discard the rest of my comment. No positive secondary effects gonna make up for that.