You absolutely MUST express at least once at night!!
Can you breastfeed them? Once they get big enough to latch on successfully it would be much easier to feed them directly. Expressing is 3 times the work, sterilising, expressing and then feeding it to your babies, breastfeeding, you just stick them on te breast when they are hungry.
Have ou got a local twins club where Mums breastfed? Breastfeeding counsellors, lactation consultants etc, can help you with the technique. It is well worth tandem feeding if you can get the hang of it.
You will produce enough milk if you stimulate the breasts enough, ie expressing or breastfeeding. It is a supply and demand thing, if you put your babies on to feed, or the pump, you will have a let-down of milk, even if it's only 10 mins after the last feed.
I breastfed my twins for 13 mths, and I know a Mum who breastfed triplets.
Have you checked out the multiple birth section of this forum, www.mumsnet.com/Talk/multiple_births there's lots of people who have done it on there.