Having failed with the Portable and with PCjr, IBM is damned if it is going to allow the Convertible lap-top Personal to go the same way, and yesterday announced its latest attempt to get the thing right. This time it has added a new back-lit supertwist liquid crystal display and an enhanced power supply to create the Model 3 at the old $1,695 price of the Model 2, which is now reduced to $1,395. Upgrade kits with the new screen and power supply cost $350. The static CMOS memory, expandable to 640KB, has a resume feature that enables the Convertible to be turned off in the middle of an application and later restarted at the same point. An optional internal modem that supports both the IBM and Hayes Attention command sets also is available.