You need to give them unlimited hay rather than putting in a bit at a time - my hutch had a layer of wood shavings and plenty of hay strewn over the top, a bed packed with hay and a hayrack with fresh stuff so it didn't get trampled (my last guinea died on sunday, only got rabbits left now ). They really do eat an amazing amount of it, because they constantly graze (and have to, to keep their digestive systems moving, otherwise they die). It has no nutritional value though, so they can't get fat on it. By an amazing amount I mean that if you put in three pig-sized piles of hay each morning, expect it to be gone by the evening - then you'll have to put more in to see them through the night! They will eat their beds around them, so you'll also have to top that up every day.
Around 80% of their diet should be hay and 10% should be fresh fruits or vegetables (check online for lists of 'safe' foods, some - like cabbage and lettuce - can be harmful, and sugary fruits like apples can cause digestive problems). The other 10% of their diet should be good quality pellet food. Don't use muesli kind, as they're too selective with that and won't get all the vitamins they need. And don't buy big bags and store them, as the vitamin C depletes very quickly regardless of the use-by date on the bag - just get small bags and transfer the contents to an airtight container. Treat-wise don't buy anything dairy-based (like yoghurt drops) or anything that looks like bird seed (like those sticks you get). Ours liked the dried corn cobs to nibble on, but they enjoy fresh greens more than anything you can buy.
When buying your hay, make sure it is good quality meadow hay. By good quality, I mean avoid any of the pets at home shrink-wrapped stuff. It's always short-chopped, pointy and hard, is a bugger to get out of the bag and never smells or looks fresh - good hay should have a slight green tinge, it shouldn't be straw-coloured. We get ours from the local garden centre now in 'loose fill' bags and the difference is amazing - it smells better, looks better and feels better, and really does show how bad the PAH stuff is. And it's cheaper! I do still buy dried grass shrink-wrapped though - I get the one with herbs in it, and they love it. It's about £4 a bag but it lasts for ages. I fill up their hay rack with it once a day instead of using plain hay.
For bedding, never use 'sawdust' (i.e. dust!) as the guinea's respitory tracts are very sensitive and it can cause terrible problems. Most of the bedding listed as 'sawdust' isn't actually sawdust at all, but rather small wood shavings - these are fine. For the same reason, don't use scented products either (you'll see stupid things like lemon scented sawdust in most pet shops). You also can't use straw as it's too hard and can poke them in their eyes when they burrow, or any type of shredded paper bedding, as this can get wrapped around their legs and cause injury. Any type of 'fluffy' bedding (the cotton wool kind) is also a no-no as it can cause blockages when they eat it. Basically, just ignore all the pictures on the front of the packets and you'll be fine - it's amazing how many dangerous products they put a GP picture on! Stick to hay and they'll be happy.
House-wise, please make sure it is big enough for three of them. They need so much more room than people realise - as a guide, you need an absolute minimum of 12 square feet of floor space (this would be a 6 foot by 2 foot hutch, or a 3 foot by 4 foot cage) and this is excluding any exercise area you provide - there's no point having a tiny hutch a big run, as they won't use it in bad weather and will be cramped inside. They don't usually do well with the ramps in split-level hutches/cages unfortunately, as they're not good climbers. Even with enough space, you might find three will fight (especially girls) so could end up having to separate them in the future. Give them each their own plastic igloo beds, and make sure the run has plenty of things to hide under (as prey animals they will get stressed if they are out in the open with no shelter). You'll need to scoop out the wet areas of shavings every day and check their beds, as they will poo and wee everywhere and it will soon get very smelly with three in one cage. They don't cope well in the cold so in winter you will either need to bring them inside or go to great lengths to weather-proof their house, otherwise they could freeze to death.
Sorry, this post is loooong! And unfortunately there is so much more to say - too much for on here. There is a lot more involved in keeping pigs than hamsters - alot of people think that caged animals are easy, but I think they're much harder to look after than cats or dogs. And so many things are harmful to them that you wouldn't expect. There are loads of guinea pig websites and forums online, so it would be a good idea to google them to read up on them.
Looking forward to hearing their names