sed to take 3 lines with same variable

Hi,

i got an input file like this
Header: One
Subject:geography
Text:passed
Header:Two
Header:Three
Subject:physic
Text:failed
Header:Four
Text:passed


I need to produce the output like:
Header: One
Subject:geography
Text:passed
Header:Three
Subject:physic
Text:failed


I need to get 3 line that containing sequences with
Header,Subject and Text

any ideas?


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