Rich,
a blank MCDU, usually only ever one at a time, will happen once in a while. Some models of the MCDU are more susceptible than others. Normally it will recover as you say, and occassionally you will need to do a CB reset. In either case, nothing to be too concerned about. (Check FCOM 4 - Degraded Ops) for more info.
I've not seen the "3WPT -1RTE" message but you'd expect to see that in the Pilot Stored Waypoints and Routes pages telling you that you have 3 WPTS and 1 RTE stored respectively. If entering a WPT causes the blank MCDU, there may be something amiss in the database or the WPT you are trying to store.
Hope this is useful
