We are using the same program to run in thin client, a web browser in PC and a web browser on a phone.
Will the screen auto resize to fit the browser or do we need to have different programs for each environment?
No, you don't need to have different versions of your program for each scenario.
You can use isCOBOL's LM-RESPONSIVE layout manager.
This layout manager automatically resizes, hides, shrinks, or enlarges controls to make the screen look good on different screen widths.
A sample is available under the isCOBOL installation folder at /sample/issamples/s-news/RESPONSIVE.cbl.
You can also find more information about how the LM-RESPONSIVE layout manager works in our User Interface documentation, under the "Creating a control" section.