Huzzah! I 'only' have twins, but:
Google your local twins and multiples club. Get there NOW to meet people and register your interest in cast offs and to get top tips eg what you need 3 of, what you can do without.
On which note, I had 72 muslins. Go figure.
Join TAMBA, if nothing else for the members' discounts you get.
Make a list of everything you need and set up saved eBay searches. eBay is your friend when it comes to multiples, otherwise you is broke my friend. Similarly buy baby-related BOGOFs now when you see them in the shops.
Get Prem size nappies off Amazon as most shops don't do them. JoJo do nice prem clothes and there is a 20% discount with TAMBA.
If you haven't got a tumbledrier, get one, and investigate switching energy supplier as you will have machines running all the time.
Start looking into maternity nurses if there's any way you can afford it and, if not, consider asking people to contribute rather than buy cuddly toys. I know you've already had a baby - so had I. And I had a loving DP. But unless you're some sort of latterday miracle you are going to have a CS and be somewhat incapacitated which is tricky enough when you have one baby to nurse, let alone 3. They help physically, they let you get the occasional 5 mins sleep, and most importantly mine helped me establish the Holy Grail with multiples which is the routine. I think I would have even less sanity left had the twins not been in a routine (and sleeping through) from about 7 weeks. One demanding baby is hard enough, but 2 or 3, whose demands will likely otherwise be on different timetables...doesn't bear thinking about!
Get 3 lifesize baby dolls from a carboot and practice with them how you'll transport your little brood round the house, how you'll feed them together, bathe etc. I have one triplet Mum who was on her own most of the time and actually made the executive decision NOT to bathe them for the first few months having done this, and just gave them a quick wipe each night. On which note, I do think a lot of multiple parenting is like that - ruthlessly pragmatic, not aiming for perfection, just to get through each day with everyone alive and, mostly, happy 
.
Everyone will be super excited about triplets - friends, family, colleagues - and rightly so and will offer to help. Rather than brushing them off, say, oo, actually...draw up a list of tasks you can delegate eg cooking ready meals for the freezer, driving round to pick up second hand cots you found on eBay etc.
And for all of this, get on the case early. As PPs have said, 34 weeks is the latest you'll go, it might be earlier, and you don't know how you'll be feeling towards the end. Even with twins I had to stop work at 30 weeks because I just couldn't go on anymore and was practically bedridden with SPD and swollen ankles and the lot. So take advantage now when you're feeling OK.
And congrats again. You'll rock it 
.