The last software ever released for the Mk3 was v3-1/63

Check what version he has loaded now in the Settings screen (top right)

V31 will update the software to 3-1/63 if he has an older version

Your modified splashscreen is for the Mk4, to change his Mk3 you need to modify the Mk3 OS - which you can do in the same way as you modified the Mk4 OS.
Remember that the CD MUST be in Mode2/Form1 and with minimum errors / maximum quality for the Mk3 to read and load the new SW