What can we do with soft clay terrains to make them suitable for construction of 2/3 story app, or do we always make a choice for a pile foundation?

 we can create under the concrete foundations rocky layer for not less than1 meter thickness and two layers2 

of enough compacted suitable base course     r

   load bearing test should be done for bearing capacity and sitilement confirmation .l 

You can use a special foundation where you introduce stab columns in to the foundation of a length dependent on where you reach soil with a safe bearing capacity. Put ground beams to carry the walls above.

Or

Depend on the area you are in ,depth of clay soils and availabilty of platforming material you can remove the clay layer replace it with suitable rravel material.

or

You varous soil stabilising technics be it chemical or mechanical eg cement stabilising or liming .