All three formats show images, but they are good at different things. Here is a simple way to choose.
JPG compresses photographs well and is supported everywhere. It has no transparency and loses a little quality each time you re-save it.
PNG keeps every pixel exactly and supports transparent backgrounds. It is the right choice for logos, screenshots and text, but photos in PNG are large.
WebP gives smaller files than JPG and PNG at similar quality and supports transparency. Modern browsers and apps open it; some older programs do not.
Use the converters to change format without opening an editor. Converting JPG to PNG does not restore lost quality; it only changes the container.