A timezone +3 icon is represented with a foldout map icon. The map will have straight sides, but on the top and bottom will be a zig zag pattern. An important aspect of the timezone +3 icon is that there will be a +3 in a circle next to the map.
This page shows the different variations of the Timezone +3 from an Icons8 icon pack. Feel free to resize and recolor the icon, and then download the icon in PNG format for free.