Can I use Golden Hammer 50% then use Vicious Hammer x2?

Does it work this way that I can use up to three hammers total?

No.

Items have two hammer slots. You can use the 50%'s twice (contrary to what their description says) or the cash shop hammers twice, or one of each.

So the 50% ones can fail and you won't get an extra slot while using up your hammer slot?

^yes

You can still use clean slates to recover the slot though.

So the 50% ones can fail and you won't get an extra slot while using up your hammer slot?

No, if the 50% fails you still get the slot, but it is used up. it acts like passing a hammer but failing a scroll at the same time.So you can still clean slate to get the slot back.