How can we know the order of the given filter ?

To quote an old University expression "by inspection" :)

Normally one RC in an active RC filter = 1 pole. So, 3Rs and 3Cs = 3 poles.

One L OR one C in an LC filter = 1 pole. So, LCL filter = 3 poles.

In each case you need to make sure you are counting just the filter components. There are exceptions. I have some filter designs with 6 resistors and 2 capacitors, but they are 2 pole.

Keith.