The idea behind buttons like this is to take away any cognitive load. It should be as obvious as possible to the user what these buttons are for, I agree that the middle button isn't clear.
I'd suggest inserting the word 'reset' in it's place. This is on the presumption that there is enough space for this.
Maybe add in your question what device/platform this is aimed at, you'll get some clearer answers off people on here.
To reply to your comment, try using the classic reset icon
This icon is well known to symbolise the resetting of an action, when placed in between two other icons such as yours, it will show the user that this is for the purpose of resetting the previously made actions.