Three boars are tough , if they've been a happy trio then hopefully this will continue.
Ask the owner which piggie is No1/No2/No3 on the pecking order.
Give them loads of space. Boars like each others company but they need Get Outta My Face space.
Boxes need 2 doors cut so that one cannot be trapped.
Have a couple of water bottles a couple of pellet bowls and seperate bowls or piles of veg.
Don't neuter them unless you want them sterile to bond with sows and don't have 2 boars together if you have sows , they'll scrap.
Neutering a boar is £40-£60 and not without risk, and if you have fighty boars with bollocks they won;t become calm after castration. (just Jaffa 
)
Boars are lovely, snuggly, greedy love sponges.
Unlike most male animals they don't smell more than females. (Actually my boar is a tidy little soul compared to the sows)
They will be shy when you get them, the only constant will be each other. But you'll win them round. Guinea pigs are bellies on legs.
Start stocking up on newspapers (they need bedding changed regularly)
Get yourself some Gorgeous Guinea shampoo (online)
Kit yourself a guinea pig First Aid Kit (I started a thread a while back)
Some piggie nail clippers
The details of a good piggie vet (they're "exotic" ) . Hopefully you won't need a vet. Ask where they usually go.
Read up on the good/bad/ugly of foods.
The owner will give you the food they're eating, if you want to change anything do it slowly.
And (not in the LEAST bit weird) ask for some of their dirty bedding to give them a familiar smell in their new house.
And -
Pigtures
MN nicknames for your guineas to preserve their anonymity 