For effective time management what's the best frequency for checking your email inbox: constantly, every hour, two or three times a day?

Well...! It could be a question where desktop or the mobile notifications are not available... Today you can set the email's desktop notification on your computers and on your mobile phones as well, so you'll always remain in touch what's coming into your inboxes. If it's irritating for you, and also you have a high frequency of incoming emails then you should check those as much as you can easily. This thing should always remain in mind that responding to emails quickly is always increase your reputation in your personal and professional circles. 

thee times a days to check the e-mail

Please check who are the senders and its urgency. If the mail is from your immediate boss or CEO, please respond immediately- this will increase your reputation. For other emails, you may check and responds three times a day- which would be quite OK.