Welcome! :happy:
First of all, if you're getting another one make sure you keep him quarantined for around 30 days to stop any nasties he might be carrying from infecting your other axies.
What are the dimensions on your new tank? My 3ft is 220L and I think 3ft is only big enough for 2 (maybe 3) axies, but I'm not sure. It's the floor space that matters rather than the number of litres - Your tank should be 18 inches long for each axolotl. 3 axolotls = around 54 inches long (although they would probably be happier in a shorter tank if it was wide).
[edit: Actually, I just figured out that my 3ft is something around the 180L mark, so I'm guessing your new tank is either 2 1/2 or 3ft long]
Sand is the easiest (and I think prettiest) substrate. Play sand is cheap and most hardware stores sell it - if you decide to use it make sure you wash it a heap of times in a bucket until the water runs clear. Don't go crazy because you don't need much - it should be less than an inch thick.
I'm not sure about filters, but internal ones do heat up the water temperature - if it's already pretty hot and you don't have a permanent cooling system then maybe steer away from these.
Have you checked these sites out?
http://www.axolotl.org/housing.htm
http://caudata.org/cc/articles/filters.shtml
As for hides, I've noticed everybody uses something different. There's no 'axolotl hide' store. Pvc piping is the cheapest and easiest, but it doesn't look very nice. I glued pebbles to mine, but it took a long time and I wouldn't recommend it unless you're seeking a distraction from piles of assignments. I think it's Becky who uses terracotta pots cut in half with the edges sanded - they look really lovely. You could try some of the hides for reptiles, just make sure they don't have sharp edges. For a more natural look, rocks leaning against each other look nice too. Have a browse through the gallery section and check out what other people have done :happy:
Have fun setting up your new tank!
(btw - LOVE the names muffin and crumpet)