You can use a ruler to measure the bolt circle diameter (BCD) as described on this page. Measure between adjacent bolt holes and then use the chart for the appropriate number of crank arms (four or five, symmetrical or asymmetrical) to make sure your crank will accept a 36-tooth ring.

Chainrings for Shimano triple-ring cranks are usually interchangeable with rings from other manufacturers, as long as the BCD matches. In other words, if you can't find a 36-tooth Shimano ring of the correct BCD, you can probably safely order, e.g., a Stronglight ring.

I see that the Decathlon pages for chainrings show BCD dimensions for the Stronglight chainrings, so you should be able to find an appropriate 36-tooth ring, assuming it's in stock and is otherwise compatible (e.g., a "narrow-wide" chainring designed for a single-chainring crank won't fit).