Your TV expects composite video, instead of RGB. The +3 doesn’t have a composite video signal, and what you’re seeing is the composite sync signal fed into the composite video input. If you can manually force your TV to accept RGB video via SCART, do so. If not, you’ll need to make a small modification to your +3 (replace R44 with a smaller-value resistor, such as 330–470Ω) for the TV to accept RGB video automatically.
Here are a few places to peruse: one, two, three.