If you have the gears like that there is too much of an angle in the chain you should really change the chain on the front chain ring to the smaller one nearer the frame. This puts less pressure on the chain when it's on the larger gears on your wheel
What you are experiencing is a problem called cross-chaining. This is caused by either the bike being in 1st gear rear and big ring at the front and also 7th gear rear and small ring at the front.
Most of this can be dialled out with a little time but because of the triple rings at the front you will never completely remove all of the rubbing.
Your local Decathlon Workshop should be able to help.
Any further questions please do not hesitate to ask.