Is monero, zcash and dash all of them using a private blockchain?
because based on my research monero is using a private blockchain to protect your privacy and make the transaction untraceable that is why monero is so popular i
in the darknet world right?
But zcash is jclaiming the same thing to make transaction with privacy and untraceable also but they are using a public blockchain how can that be possible?
Thanks in advance

Well I think Monero is the best coin out there for anonymity but I don't know about zcash. It doesn't offer that much of anonymity when compared with monero. On the other hand, Dash is good too but it ain't the best coin to use to give yourself anonymity on every transactions. As far as I know most of those cryptos offering anonymity like monero and dash does it the same but only differs in speed and traceability. There are also a lot of coins out there offering the same feature as them although they are new or is still in ICO stage.
They don´t offer the same. One key difference is Monero is anonymous and private by default while the privacy in Dash and Zcash is optional. I believe this can be subtle but is a key diference making Monero superior on that end.
Exactly well said. Many hollywood celebrities are already using monero so the IRS won't be able to trace there income so they won't have to pay higher taxes.
Those many (or few, depending who you ask) Hollywood celebrities are not using Monero to avoid paying higher taxes, they use it as publicity stunt or to attract more users.
Do not think using an anonymous cryptocurrency will fool the IRS, they have tools available to get the information they want or need :
https://pocketsense.com/irs-monitor-bank-accounts-9978.html Attempts to deceive the IRS can carry heavy fines and even a potential jail sentence, depending on the severity.
I am myself involved with an anonymous cryptocurrency (Dash), but i fully intend to pay the necessary taxes, the risks (severe fines, jailtime) outweigh the rewards (paying less taxes).