Apart from the Kaloudis I don't know of any other 3 bedroom apartments, but that's not to say they're not there! If the bedrooms are big enough in a 2 bedroom apt I'm sure many owners would be happy to supply an extra fold down bed.
As Val n Bill have said, the Makris has two large two bedroom apartments. One on the lower and one on the upper floor. The staircase to the upper is rather steep but there is a safety gate at the top.
I've only been inside the upper apartment which is fairly spacious with a sitting/dining kitchen area, double bedroom and second bedroom which has 3 beds but still with plenty of room to move around.
makrisapartments.com The pool and bar area serving snacks is ideal for children. The Makris family are always around to look after their guests in a friendly and quietly unassuming manner.
Giannis Bardis's apartments are closer to the beach and facilities in Arillas, but don't have a pool. They are studios but some have interconnecting doors for larger families. They are gorgeous apartments and like the Makris are very reasonably priced. Giannis has the cutest tiny little dog whom I'm sure your grandchildren would love to bits!
Nikos Kokkinos spacious apartments (again without pool) also have two bedrooms with seperate living area, I'm sure Nikos would provide a folding bed for an extra child. Nikos, like Giannis Bardis, is a lovely lovely man and can't do enough to ensure his guests have a wonderful stay.
http://www.kokkinosapartments.com/accommodation/There are a few other 2 bedroom apts in Arillas which you can find in Dimitris's directory here..
http://www.arillas.com/info and others on here...
http://www.greencorfu.com/accommodation-apartments-arillas/