I need thunderbolt 3 male to thunderbolt 2 male cable to connect iMAC as monitor for mac air

Nope, it will not work, period. There are no work-arounds. As mentioned, please carefully read the Target Display Mode article.

My suggestion is to buy an external display to use with the MBA, this is far easier, far more reliable and overall less expensive (does not require expensive cables) than Target Display Mode ever was.

You need to tell us what model year iMac and MacBook Air you own and the version of Mac OS installed on each. I suspect you are chasing your tail as Target Display Mode has been dead for years now. Please carefully read Target Display Mode (Target Display Mode) to see what does and does not work.

5k imacs never could work as monitors in target display mode

last macOS that supported connecting to a target display mode enabled imac was high seira