Each format has its job. JPG is the best for photographs, where it compresses a great deal with barely visible losses. PNG keeps every pixel exactly and is the only one of the three supported everywhere with transparency: it is the choice for logos, screenshots and graphics with text. WebP is the most modern, typically weighs thirty per cent less than JPG for the same result, and is supported by all current browsers.
The most frequent conversion is WebP to JPG, because it's common to download an image from the web and end up with a format an old program won't open.
One thing to watch: when converting to JPG, transparency is replaced with white, because that format doesn't handle it.