Deleting messages only once on IPad IPhone, and mail account
It sounds like you are using a POP3 type email account. Unfortunately, using native (built in) apps, I don't know of any way to sync across your devices. Deleting emails multiple times is indeed something that can be a major pain.

One usability thing you probably use all the time, but just in case you haven't seen it is the fact that you can tap the edit button on an iOS device (upper right) and tap the emails listed to quickly delete emails. This is faster then swiping each email.

Also, if it is possible (if the Comcast email is just personal and you have a choice where to host your email), you could switch to a Gmail type IMAP service. This automatically stores all your email in the cloud and will sync across all your devices.