If you are using PBP, then I question why you are using PBC instructions like 'SYMBOL'?

I have absolutely no idea what you are trying to achieve, but the code below will at least do something... whether it is what you want is another matter.
Code:
	LED1 var PORTB.0 'PIN 6
	LED2 var PORTB.1 'PIN 7
	LED3 var PORTB.2 'PIN 8

	SW1 var PORTA.0 'PIN 17
	SW2 var PORTA.1 'PIN 18
	SW3 var PORTA.2 'PIN 1

	TRISA=%00000111
	TRISB=0
	CMCON=%00000111

	LOW LED1
	LOW LED2
	LOW LED3

	PAUSE 5000 'TIEMPO DE ESPERA PARA ARMAR LA ALARMA

LOOP:
	IF SW1 = 1 THEN
		PORTB = %00000110
		Pause 3000
		ENDif

	IF SW2 = 1 THEN
		PORTB = %01011011
		Pause 6000
		ENDIF

	IF SW3 = 1 THEN
		PORTB = %01001111
		Pause 8000
		ENDIF

	PORTB=0
	GOTO LOOP
	END
I don't know how you have your switches connected, but you should ensure the PIC pins on PORTA are not left floating at any time.

Next time, if it is PBP you are using, post your question in the Mel PICBASIC Pro category as a New Thread.