How to sync my email between three devices?

You don’t need to do anything special on your devices. The only requirement is that the email accounts be of the IMAP type. IMAP/Exchange accounts “reside” at the server, so all ”clients” on your devices will show the same synced status.

If your accounts are of the old POP type, change them to IMAP. Log into them via a web browser to make the switch and find out how to set them up on your devices. Archive your POP emails first.

if the mail account if of the type IMAP or Exchange that will be the default behavior on any device the account is setup on

if the mail account is the old pop3 there is no way to keep the clients in sync

While your smartphone isn't an Apple product, Continuity likely won't work

with your Apple devices, to fully sync the other Apple services.

But with an iPhone, Continuity could. + Mail would sync

between all Apple products ~ as would other features.

• Use Continuity to connect your Mac, iPhone, iPad, iPod touch, and Apple Watch - Apple Support

The level of hand-off and other shared features is rather extensive if they

can see each other in proximity. Even without Continuity, newer devices

by Apple can do a fairly good job of connectivity... My three fairly recent

Apple products (iPad Mini, iPhone5c & Mac mini i7 2.3 quad server) all

could share much more than I allow them, just in settings ~ plus iCloud.

As for Samsung, not sure if there's an easy way to make that fully sync;

unless there's a third party software or service that can do this function.

{And haven't researched other possibilities; while I use gmail solely.}

In any event...

Good luck & happy trails! 😉