Hi Gaurav,
ddrdushy1 is right.
On Mainframe Terminal there are 24 function keys (from PF01 to PF24).
Send Control Key actually sends those functions, not the PC Keys.
So typically the VT3270 terminal PF01 to PF12 keys are mapped to F1 to F12 PC keys and VT3270 keys PF13 to PF24 are mapped to Shift + (offeseted) F* PC Keys.
Thus:
PF13 = Shift + F1
PF14 = Shift + F2
PF15 = Shift + F3 ( and this is what you need)
…
PF24 = Shift + F12.
(These can be custom remapped in terminal emulator, so when in doubt check Keyboard Mappings in the Terminal Emulator configuration/help)
But the long story short, you need to use Send Control Key with F15 in your case.