Writing hex to several microcontrollers at the same time via ICSP
It is "possible", but not a good idea.

The Microchip PIC programming interface is synchronous serial with the programmer controlling the clock.  Strictly writing data to a PIC can be done to multiple PICs in parallel because each PIC only looks at the clock and data lines.

However, proper overall "programming" requires reading information back from the PIC.  This can't be done with multiple PICs in parallel.  During readback, the PIC drives the PGD line actively both high and low.  Two PICs sending back different data would cause a collision.  This could even damage either or both PICs.

Readback is important to verify that the right PIC is out there, and that the data has been correctly programmed.  Good programmers will also check the Vdd level to make sure it's within spec for the programming operation.  It could be lower than intended due to the circuit drawing more power than the programmer can provide, for example.

To program multiple PICs at the same time from a single PC, use multiple programmers.  I know some customers use multiple of my USBProg2 in exactly this application.  Multiple instances of the PIC_PROG program can be run simultaneously, and each instance can be directed to connect to a particular programmer by name.  By default, each programmer has a unique name based on its serial number.

Also, the PicKit3 is not a production programmer.  It is aimed at hobbyists and optimized for low cost.  Since you're asking about programming multiple PICs in parallel, you're in a production environment.  The cost of just one problem will outweigh any savings by cutting corners on the programmers.

If you ever have this need with one of the newer 3.3 V PICs that can enter programming mode using a special signature instead of raising MCLR to a high voltage, you can use my LProg programmers.  These are small and cheap, and are production programmers, but a little slower than the USBProg2.  The PICs it supports include all the 16F1xxx parts, and many others.