Yes, you can indeed reduce the icons down to three if you prefer, although we do generally recommend sticking to four icons if possible as this ensures that each response is clearly either positive or negative.
Having just three icons would mean that there was a 'neutral' option, and if the recipient were to click that it wouldn't really give much feedback about their interaction with your company.
There are a few different ways to remove icons depending on the Thermometer type. I've listed the relevant two for you below.
You can remove an icon by going to the 'Thermometer Setup' page, scrolling to 'Name your temperature ratings' and removing the description next to the icon you do not want to appear. Then click save and preview your template to ensure you have removed the correct icon.
You can remove an icon by editing the HTML code by finding the section which refers to the icon you would like removed and delete it.
Please note, removing the Green rating will impact your NPS calculations.