The first question is easily handled by triggering all instruments at the same time and then querying each in turn for the measured value. You can use eaither a hardware or software trigger (i.e. the gpib GET command). For your second question, are these 3 LabVIEW programs started by a single main program? How you want to run them will make a difference but in any case, the programs will have to have some sort of synchronization mechanism if you want measurements to be taken at the same time or if independent, a blocking mechanism to prevent simultaneous reads and writes. More details on the second question would help.
