[The Lucas]

Here are a couple things (differences with my code) to try :

Try declaring Old_Port_B like this:

	unsigned char Port_B;
	unsigned char Port_B_Delta;
        static unsigned Old_Port_B = 0xFF;

Try spliting your handler into separate rise and fall handlers like so:

		if (Port_B & 0x10) Int_3Rise_Handler();
		else Int_3Fall_Handler();

Double check that you set INTCONbits.RBIE in initialization
Those are the only differences between my handler (2 encoders & 3 GTS) and your handler. We changed it last year and I don't remember exactly what fixed what.