The waiter asks them what they'd like to drink. The first one asks for a lemonade. The second asks for a coke. The third asks for lots and lots of beer.
After the waiter has brought them their drinks, he takes the order for their meals. The first one asks for steak. The secons asks for lasagne. The third asks for lots and lots of beer.
After they've had their meal, the waiter comes round to take their dessert order. The first one asks for ice cream. The second one asks for chocolate cake. The third asks for (you've guessed it) lots and lots of beer.
Once they've paid their bill, the waiter asks the third pig why he's just been drinking lots and lots of beer all evening and the pig replies (this is brilliant) 'Well, someone's gotta go wee, wee, wee all the way home.'