I am trying to build a project using PVC pipes. How can I get SOFT Core PVC?

Depends on the size you need. Larger PVC is typically foam core, what I would consider "soft" core. Smaller sizes are solid PVC. Don't think you will find foam core in sizes less than 1", maybe larger. It is a bit more flexible and lighter. What is your project? HD and Lowe's have foam core.

Need 3.0 inch dia.

HOme Depot Website
JM eagle Model # 53723

Internet # 100173538

Store SKU # 295096

3 in. x 10 ft. PVC Schedule 40 DWV Foamcore Pipe
$10.78