They can't. Unless they agree to a major haircut for chips.
Incorrect. They can go over the cap with Bird rights to sign these guys. The cap hold number for free agents tends to thwart these types of things (to keep Bird rights to have to have the cap hold in place as you sign other free agents), but the cap hold number is based on prior contract and Curry's is ridiculously low. This gives GSW tremendous flexibility (that, the cap spike, luck, and tremendous foresight/planning). Supposedly, Durant will take a little less than the max, probably in a 1+1 (player option on the second year) so they can sign him with cap room and hang onto Iggy/Livingston via Bird rights (although both/either could end up leaving). Then Durant opts out again next summer, and resigns for a full max with Bird rights. Doesn't hurt that Klay and Green took slightly less than their maximum contract values when they re-upped a couple of years ago. Maybe they lose guys like West, McAdoo, McGee, and Zaza, but they're largely replaceable, especially with ring chasing vets willing to latch on for a year.
Their luxury tax bill will be effing astronomical in 2 years, but they have a new arena coming (which I think they own outright), and they bought the franchise for ~ $450m in 2010. While that was a record at the time, the franchise is probably worth $3 billion now; that's pretty good appreciation. And as a two-time champion with likely more coming, they could be in the midst of one of the greatest stretches in modern sports history, which means revenue (Game 5 alone was ~ $22m revenue, $11m profit). All of which is to say that they should be fine re: money.
Buckle up - it's going to be a good run for GSW fans.