we did not use the impact3d software. on the flexlm server find the ptc.opt option. set the value TIMEOUTALL to 1200 seconds. This flushes all licenses every 20 minutes. in you config.pro set the option SUPPRESS_LICENSE_LOSS_DIALOG to yes. this will suppress the regain dialog box that you will see after setting the option on the server.
Like Proed mentioned your best bet is to go with a customized .bat that semi-automates the task. We use a .bat that generates a menu of all of the used licenses that we can pick from and flush. It works great!
Is your license number actually "Flex3C"? This should be the actuall license number that shows when you type ptcstatus. It's not the license 'type' but the actual number. All this batch routine does is types the ptcflush command out for you. The syntax needs to be exactly as you would type it yourself. If you can get ptcflush to work, make sure what is in the batch file matches what you hand type.